Dakar, one thing to remember is this. If the ball isn't hit to you (for what ever reason) you can't make a play on it (Put Out/Assist).
If you have some major Ground Ball Pitchers your entire OF will lose out on Put Outs as less OF Flies will occur. Since OF Assists are pretty rare (compared to other Plays, like OF Put Outs, IF Put Outs and IF Assists aside from 1B Assist which may be even more rare than OF Assists) they don't have a lot of room to make up those Lost Put Outs.
If you have a lot of Fly Ball Pitchers you will notice the IF will have a lesser RF than usual, and the OF will have a higher RF.
RF is less an indicator of how great a fielder is, and more indicitive of what kind of Staff you have.
Stadium Effects matter here as well. Having a Cavern for a Field means less HRs... which means more Fly Ball Outs. Having a ton of Foul Territory does the same thing. So Park Dynamics can also impact RF (more for the OF but also for the IF where Foul Territory is concerned) as your Corners (1B, 3B, RF, LF) will get a few more chances at Put Outs this way.
There are only 27 Outs in 9 Innings, so those little variables will matter over the course of a Season. It's not so profound as to add an entire 1.00 to a RF, but it will matter. This is why you may trade for a RF with a 2.40 RF and end up with a RF of 2.20 due to having a Shorter Fence and less Foul Territory. He's making all the same plays he used to, he just has less ground to make the plays in.