Ian talked about this a while back in one of his blogs.
On the lower levels of difficulty the seperation of the sped of players becomes more and more aparant, while on All-Pro and All-Madden he speeds are supposed to be sim, or realistic. In Rookie and Pro a 99 speed guy should just LOOSE a guy only a few points slower and so on. The same holds true for other ratings as well. 89tha in Pro should mean you can hit most any pass with your QB but the computer QB should have a hard time. on All-Pro and All-Madden 89tha should mean that you have a good shot at hiting you man, but the ball will have some woble to it and may be a little of target. Too low, too high or even behind the reciver, also known as thorwing where your reciver was adn not where he was going.
I always felt on All-Pro that ratting began to really matter in the passing game and play recognition the most. Everyone has talked about how on All-Madden the computer players/animations/collision detection gets so scewed that you are fighting two battles. one against the computer and one against the other team *controled by the computer* I can't count how many passes have gone through my defender's adn reciver's bodies/hands/etc on All-madden that would alter the entire make up of the game. Is that pick? no a TD becasue the computer cheats me adn the ball goes through my guy. A wide open first down on third and long to kep my drive and the game alive? No! Dropped.. err. went through the player/ball thrown poorly into the dirt even when I had my fet set and solid timming on the pass button. All-Madden, in the past has done many questionable things to hurt you and your teams chances to give the computer a chance/advantage over the fact that you are a REAL perdson with real skills.
I know Ian has and will continue to deny this, and maybe it isn't true anymore but we have all experianced it and we all know it's garbage.