This seems like an anomaly for some reason. Robert Williams is simply not that good. I am not sure he is even more valuable than Tristan Thompson. To me, Williams is a solid rotation player who is a fringe starter. Certainly could start on many teams, others he wouldn't.
He doesn't quite have the skills to play as a PF and doesn't quite have the size and bulk to play as a center against some of truly big centers. He does have a nice feel for the game which leads to nice dunks and blocks. He is part of our serviceable center 1-2 punch and does provide a nice contribution in that role.
I like RWill. He has worked hard to develop to the point he has. He has been fun to watch and root for. But if the PER stat is suggesting he is a top player in the league, even for the limited minutes that he does play, there is something amiss with the calculation.