Think about it though, 85% of the rebounds in this game are defensive rebounds (i.e. come from missed shots). If they are shooting the NBA AVG for FGA 81, and hit 40 (near 50% FG%, which is what it was with 3.5), there are only 41 rebounds available, 85% of that is 35 defensive rebounds. That is all that is available defensively if a computer opponent shoots just under 50%. If they shoot over 50%, less are available. They don't just disappear, and the computer will always come in with 15% ORR over a period of time.
It will straighten out as the CPU starts shooting worse and they will.