RE: Take Kyushu PBEM vs AI
Mostly coincidence I think.
Unless I missed it, there's no solo version of Take Kyushu with different US unit ratings.
The AI is likely to be poor at focussing fire compared to a human opponent, which means your defenders will be in a better shape.
The first wave units disrupting can be quite a pain for the Allies, as their HQ takes a while to arrive.
With delayed disruption reporting on, it can be difficult to keep track of what happened when as the only direct feedback of enemy disruption results comes from disruptions that occur during the enemy turn.