With Bynum, I think he has always been a great raw rebounder, but he was still developing a bit, and couldn't stay on the court long enough to get a real rhythm going. So, I think we are just seeing the effect of having him be healthy, as well as just natural progression.
With Perk, I think it has more to do with the system. The C's system really is not conducive to high rebounding numbers for big men. It requires the big men to spend a lot more time blitzing the perimeter, and to do a lot more helping defensively, which often leaves them out of position for rebounding, forcing the perimeter guys to take a much larger role in that area. This has always kind of kept Perk's rebounding numbers a bit lower than they could be.
However, in OKC, their system is more traditional, where the big men really do stay down low more, and they really do rely on them for the rebounding a lot more.