The mathematical effect identified by Krupicka is probably more significant than the one identified by Chuck, and probably more likely to explain the difference at this tournament.
You can see the same ratings skewing effect in rounds 1 and 2 of the Stateline this past weekend, comparing the ProAdv pool to the Int pool. Par averaged 932 for the lower pool and 950 for the upper pool. Same courses, and over two rounds, same average conditions.
Circuit court staff attorney - 25 years. My judge must retire. Looking for employment.