AP seems to change mostly based on;
a)difficulty level (I think AI usually has LESS AP up to ~hard or NM solos?)
b)base AP of player units.
The only time I see a decrease is when facing Undead AI. The increased base AP costs of UD seem to really limit them from bringing extra units, and the gear assigned doesn't cover the difference. In suicides they still bring more AP than I do, but less than any other race. Maybe this is affecting your averages?