[Trebor the Mad Overlord]

My impression from watching PH on TV is that he is better than almost anyone when playing against weaker players, but is not nearly as strong when playing against world-class players (ie: on HSP).

This means that he has a bigger edge in larger tournaments with a lower average skill level, which is why he seems to do better in WSOP events - the huge fields make for a happy hunting ground for his particular skill-set. By the time he becomes a negative-expectation player vs. the remaining field, he's deep, probably in the money, and probably has a big enough stack to survive for quite a while.