I can't say for sure how EA designs the game on different difficulty levels, but it's a definite given that most sports games seem to not follow the above theory; instead, attributes are bumped up to make the CPU more difficult.
This is most telling in baseball and basketball games. That's when you start seeing the CPU AI hit 60% of their shots, contested, falling away. That's when you see guys blasting homerun after homerun even off the toughest of pitches with ease.
And yes, that's when sports gaming is most frustrating. It's what has mostly turned me away from sports video games.
I can't remember which basketball title it was, but I remember a developer once saying that at their highest difficulty levels the attributes aren't bumped, rather reaction time and intelligence was bumped. So, it wasn't that Kobe can suddenly hit 60% of his jumpers. It was that Kobe could better recognize open space and create a shot. Or it was that Kobe can better close the lane preventing you from driving to the hoop.
That's the way higher difficulty levels should be tuned. Again, I'm not sure what the case is with Madden, but I guess if enough people feel that that isn't the case then the development team would need to continue fine tuning it.