Most of what anyone thinks they perceive in these kinds of tests, whether it be on frames or wheels, is a psychological construct of acoustic feedback. That's pretty much it.

If you could deafen yourself, and blind yourself, you would find that humans are mostly incapable of "feeling" the differences between wheels, if all the wheels being compared are properly made and functioning. Typically, differences in vertical stiffness amount to the thickness of a sheet of paper. If there are lateral flex issues, then the wheel was improperly tensioned, or has too few spokes for your weight.

That's pretty much it.