The reason he looked like a good rebounder is his MPG was much, much lower in his earlier years. He's just not a talented rebounder :\
MPG:
2006: 9.2
2007: 7.7
2008: 19.7
2009: 19.4
2010: 13.0
2011: 26.1
2012: 31.7
His Per 36's looked good when he was only playing 9 minutes and getting 2 pts & 2 reb a game.