Your logic is backwards.
Devin Hester didn't lead your league in receiving because he was a 90 OVR WR with the Speed scheme setting. He led the league in receiving because you set your team to value Speed WRs, which resulted in you (or the AI) placing Devin Hester at the top of the depth chart because that scheme setting really values Hester's elite physical ratings (as reflected by his OVR rating being adjusted greatly upward). Thus he had a ton of opportunities to catch passes and clearly you as a play caller found ways to get him the ball if you led the league in receiving yardage with him.
The OVR rating simply doesn't affect the football gameplay, it only affects what players get placed onto the field based on the auto-reorder depth chart operation and players' contract demands. The scheme setting you pick doesn't affect the individual ratings which govern how Hester plays on the field - it doesn't change his catching, his route running, his awareness, and so on.