Whether it happens because of a CPU rating boost or a user rating reduction, it's safe to say the CPU has an advantage on AM.
If you need proof of this, go into practice mode, take note of the matchup stick on a particular play, then lower the difficulty to all-pro or lower (it will REALLY stand out on rookie), run one play to complete the difficulty change, and note the matchups again. As you go down the levels, more of your yellow matchups should go green and at the very least, the Speed and Route running numbers should change.