Clipping could be reason for WHM,if they're DPSing heavily. The thing is, using Divine Bension and such really clips into your GCD if you're not using it after Aero. I'll gladly clip it for a Thin Air or PoM, but Divine Bension and Asylum are sadly things that I'll put off a bit at times because it clips into my GCD.

SCH and AST don't have this problem with clipping, so that's probably why you notice skills being used more on them and less on WHM.