I have to think it's a combination of health, minutes and Perk.
1) Obviously the knee is an issue
2) KG played < 30 MPG for the first time since his rookie year. He used to be a 40 MPG guy.
3) Perk is pulling down about 2 RPG more these days than when KG first arrived
Obviously he's not the same KG who used to average 13-14 RPG in his prime, but if you give him a weak rebounding center and longer minutes he could probably still pull down 10-11 per night.
I don't suggest the latter, but with Perkins out and KG's knee possibly another notch recovered, I wouldn't be surprised to see him back at the 8-9 RPG level this year.