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Part of it is down to faults in testing methodology from a lot of people and part of it is them trying to convince themselves that they've not just blown $250 on an antenna which works no better than the $50 one it just replaced. A lot of the wording in the posts in this thread and others in "is X better than Y"  and "if I do this how much better will that make it work?" pretty much give the impression the poster is trying to convince themselves they've not just wasted a bunch of money.

Like you I've done a lot of antenna modelling and I've built a lot of what I've modelled. The real world results I've seen from the various designs and alterations to those designs have convinced me that the models are sufficiently accurate enough that we can take the comparisons of various CB models like you've done and that the differences illustrated between those being compared are going to reflect the real world. Any large differences are therefore down to the fact there's been a change, either a previous unrecognised fault has been rectified, some condition has changed such as using a different length pole and thus changing the height of the antenna which improves or worsens local contacts or in the case of DX that the ionospheric conditions have changed.

Some of the gains claimed are just quite ludicrous. 6dB is a fourfold increase in signal and 6dB difference is damned hard to the point I'll say it is impossible to achieve from one antenna to a similar antenna - i.e an Imax 2000 to a Vector 4000. Some claim several S points but that's just ridiculous. Here's what an increase of 6dB looks like and its certainly not several S points and to get that 6dB increase I had to double the size of the antenna. I certainly didn't increase it by just a few percent as is the difference between a 5/8 wave and .64 wave.