First off just where do the claims / specifications come from?
Just how were they derived?
And last but not least what does the claimant have to gain ' financial' or protect 'ego' by the claims made?
I will leave those questions for you to answer for yourself. :)

Their is a old saying, ' The performance of a item is directly proportional to the cost of the item or the higher the cost the better it will perform '.
There is some truth to the above statement, however the term performance is a very wide brush when it comes to describing any item.
The mechanical survivability 'performance' of a antenna that is well designed structurally will be better than one of a poor structural design, this says nothing
however about the antennas 'performance' related to its ability to receive or radiate a signal, and that my friend is one of the 'performance' areas we as amateurs are most interested in.

If you see a claim 'specification' that states a antenna has xx dB gain without a qualifier you have (NO usable information) about that antenna.
The term dB is always a ratio of something to something else. For instance a circuit with 3 dBw gain means for a input to the circuit of one watt you will get two watts out of that circuit, this is not exact but close enough for this article. If however the 'specification' left the 'w' off (just dB) then you have no idea what the gain is referring to, and thus (NO usable information) about the circuit. The same applies to antenna gain specifications.

Relative power in dB = 10 log10 (P1 / P2) where P1 and P2 are two power levels measured in the same units (e.g., watts).

gain dBi

An isotropic radiator is a lossless dimensionless point in free space that radiates equally well in all directions. One of the chief advantages of the isotropic radiator is that its field never changes, so that it functions as an agreed upon constant against which every antenna may be measured in every direction. Every antenna measurement referenced to a P2 that is the far field power from an isotropic radiator has a positive or negative gain in dBi.

gain dBd

dBd compares the gain of an antenna to an theoretical ideal dipole in free space. An ideal dipole uses infinitely thin lossless wire and is resonant at the frequency of interest. In this application, the ideal dipole has a gain of about 2.15 dBi, that is, 2.15 dB over an isotropic radiator. All measurements are thus arithmetically transportable between dBi and dBd by adding or subtracting 2.15, as appropriate.

NOTE: dBd can also be expressed as the gain over a real dipole set in the same position as the test antenna, where both antennas are oriented for maximum gain
relative to the far field receiving site of the test range. For fairness, one should specify the design frequency and construction of the dipole to ensure that the materials are comparable to those of the test antenna. OK enough rambling, back to the question at hand.

Hypothetical example:
Ham number one installs a new super dipole antenna at his antenna farm, it has a purported  gain of 30 dB boy is he pleased!
Ham number two gets the dimensions from number one and installs the antenna at his antenna farm, man this thing is a peace of garbage, he gets
better reports from his old antenna. What went wrong? Did ham number one lie to number two?

Not really, ham number one gave number two 'dB gain' (No usable information) about the antenna in question. Also antennas are effected by the environment in which they are placed. What you say! Well first off number one used the 'S meter'  on his radio for the measurement, and we all know how well they agree between makes and models of radios, he also lives on a plato at 1200 feet asl and number two lives at -50 feet asl. Second number one placed his antenna 100 feet agl and number two placed his antenna at 20 feet agl. Third ham number one ran his antenna north south while ham number two ran his East West. To make maters worse they used the same station for the signal referenced on different days and they are only 30 miles apart. Talk about the way NOT to compare antennas, that was it.

Some things you should know about antennas:
All antennas are effected by the height above ground, the quality of ground at the frequency of interest and all obstructions in the signal path of interest.
The more efficient the antenna is at the design frequency, 'that is the ability to radiate and not the apparent band width', more on that later, the more it is effected by the height above ground. This ground effect can and does take many forms. First the EM radiation  from the antenna can be enhanced or reduced, (even the polarity can change) with height. Second the match (VSWR) or load resistance as seen by the feed line can and is effected by height. The rule of thumb is the closer the antenna to ground the lower the feed point resistance, that being sed that is not always the case so don't count on it. Most often the change in resonance of the antenna is not taken into account when the height of the antenna is changed, but it does and sometimes by a large percent of the design frequency.

Most antennas used by the average ham are loaded down with compromises. You can have a efficient radiator at a narrow frequency range or a wide frequency range with less efficient radiator but not both. The most often found compromise is the HF dipole. It is tuff to get a 80 meter dipole 1/2 wave above ground let alone a 160 meter one. As I stated before the closer the antenna to the ground the lower or the higher resistance it presents to the feed point, which is due in part to the some of the EM waves (we as hams like to call it RF) see notes at end of article, being absorbed or reflected by the ground instead of being radiated as we would wish.
Second is antenna system loss, or maybe this should be the first case. This includes (in the case of the dipole) wire type and size, coils, traps, chunks of coax, resistors and other items used to make the dipole 'flat' from DC to LIGHT, "that is to increase the apparent bandwidth". Take for instance the terminated folded dipole, why do you suppose the terminating resistor MUST be rated to handle a minimum of 1/2  the power applied to the antenna? I will leave that one for you to ponder. :)
If you think traps are loss less tell that to the amateur who just burned the traps out of his/her antenna while using less power than the antenna was rated for.

So in a nutshell (don't believe all you read, even this article .... ..  .... .. ). Try and determine what if any standard was used to derive the antenna specifications and last but not least, if your ham friend gives you a antenna that works wonders at his location do not be surprised if it is a dud at yours, remember you may just   need to re tune it, or it could have been a dud all along. :)

NOTES:
abbreviations used in this article
log10 =  log (base 10)
agl = above ground level
asl = above sea level
vswr = voltage standing wave ratio
dB = decibel
HF = high frequency
RF = radio frequency
EM = electromagnetic waves

All electromagnetic, EM, waves, traveling in free space, have an electric field component, E, and a magnetic field
component, H, which are usually perpendicular to each other and both components are perpendicular to the direction of
propagation. The orientation of the E vector is used to define the polarization of the wave; ifthe E field is orientated
vertically the wave is said to be vertically polarized. Sometimes the E field rotates with time and it is said to be
circularly polarized. Polarization of the wave radiating from an antenna is an important concept when one is concerned
with the coupling between two antennas or the propagation of a radio wave.