I don't think he's an underachiever or that he put up bigger numbers because it was a contract year. Every time I've seen him play since he was a rookie he has been one of the hardest workers and biggest hustlers on the court. I think he's just figuring out how to play in the NBA. He was bigger, stronger,and more athletic than most college players. He doesn't have any of those advantages in the NBA, and he looks shorter than 6'9".
The hustlers like Humphries who pound the offensive glass are usually my favorite types of players.