Its an interesting observation because statistically he WAS much better his rookie season (4.9rebs in 16.9min = very respectable). But I think a couple of things can be pointed to there:
1) in his rookie season, due to his lack of size they were trying him a lot at SF, where obviously he would have a prohibititve advantage
2) his real potency was on the offensive glass -- he wasn't much better on the defensive boards, which are not as sexy as o-rebs but considerably more important to a team to finish off defensive plays
3) the relative fall off in his offensive rebounding (he's still better there than on defense) might be because he shoots more jumpshots. Its possible. Or it also might be because he's not boarding over SFs anymore. Or maybe its because he is playing far more minutes and can't bring the same manic energy. Or maybe its because not only is he going against PFs, but now starting PFs and Cs often approaching 7'0".
In any case, even at his best, and that best wa a 42 game stint averaging 17min per as a rookie, you basically had a guy who was good at crashing the offensive boards to get himself extra interior shots. Be a nice trick to have (and again he's still pretty good at that), but never, not even during his rookie season, did he ever show the ability to ovecome the size issues and control the glass defenisvely.