Here's what it looks like to me. The overriding factor is the random difficulty setting. A better ovr may have a higher chance at getting a favorable difficulty level for that quarter. I see it as four parts to the different difficulty settings that affect and contribute to the final score:
1. Your manual quarter - how your players will play and how the AI players will play
2. Your opponent's quarter - how your opponent's players will play and how your AI players will play
If you're not jumping for rebounds, missing perfect releases and can't steal anything, that's because it has been pre-determined when you started the quarter. No amount of defence you throw at it will be able to stop the AI's 100% field goal percentage, if you're unlucky enough. There is no way to beat the OP AI, because your players can't move even when you are mashing those controls. There is no skill involved in playing this game, just luck.