Sully pulled down 8 rebounds in 7 less minutes per game than Okafor. Per 36, Okafor grabbed ~8 rebounds to Sully's ~13.
Yep, your comparing a rookie to a guy who has been in the league a few years. Perhaps you did not understand that. One was a rookie and one was not. Normally, people improve from their rookie years.
You're right, comparing Okafor to Sully this year isn't fair. But looking at Sully's rookie season, he still pulled down 10.7 rbd/36, which is much better than Okafor's 8.4 rbd/36. Even Kelly Olynyk managed 9.4 rbd/36 his rookie season. Okafor has his strengths, but he's a bad rebounder.
Rebounding rates don't usually increase significantly over a player's career, although with a reduced offensive role Okafor would probably see a modest increase (maybe enough to make him average)