The data collected from the same barrel as it is cut shorter is far more reliable
while it is reliable, it's not really any more useful, as the barrel it came from is now in little chunks.
People who know, know, the rest usually don't think about it, until something points it out. There are fast guns, and slow guns. Its all about the individual gun and the ammo being fired, and how they "fit" together. A different gun, even the next ser# off the production line can be an entirely different thing.
Most are pretty close to each other, but both ends of the performance curve are out there. I have seen 100fps difference between three 6" barrel guns firing the same ammo. That much difference isn't common, but it can happen.
A 2" barrel shooting faster than an 8inch is unlikely but has happened as well.
Any claimed velocity may be 100% accurate, for the gun it was fired in. It may, or may not be what you get from your gun and ammo, even if your equipment is as closely matched to the test gun as possible.
My experience is that when talking velocities over a range of guns in a given caliber, any claimed speed different of 100fps or less is well within the normal variation found between different gun and ammo combination.