Well, it would be simple math and as such easy to understand if USTA published their algorithm... but of course they don't, and for good reason, so we are all left to speculate on how certain results came to be. Some people's guesses (like schmke) are perhaps more educated and better than others', but ultimately still a guess.
Also, just because the current algorithm kinda works does not mean that it is the most optimal way to calculate ratings. So it is fair game to challenge it. For example, I happen to think that an ELO-based algorithm that adjusts a player's rating after every set would work even better.