It varies a lot because of the difficulty of the questions. I got 230 on UWSA2, which was about 65%, and then after another two weeks of prep, I got 228 on NBME 11, which was about 83%. It's clear that UWSA questions are much harder and the curve is much friendlier.
So, if you're especially good at hard questions (say, 65%) but not so good at easy questions (say, 70%), UWSA will overpredict. If you're especially good at easy questions (say, 90%) but not as good at hard questions (say, 50%), UWSA will underpredict.