This is my 'theory' on the whole coach rating thing.
The ratings might only be for simulations, since if you use Association and play it yourself, you're the coach.
Offensive Ratings;
-The coach will have a better offensive strategy and tendency.
Defensive Ratings;
-The coach will have a better defensive strategy.
For the others, I don't know.
But this often just leans on the simulation strategy (which will be shown before the first game of the association season in year one)
Remember, that's only a THEORY.
Not really an explanation, but it is probably what's happening.