Glitch in the game... As a knight (I'm not sure if this happens with other character types) my mount will increase my health. Like as a level 6 with an "Epic Donkey" my health was 113 I believe. Then, I will level up and my mount health bonus is gone. I had 57 health at level 7. If you could get that fixed, it would be great. Awesome game though, I love it :)
The theory that the AI has the same units is false. I tried it and ended up against longswordsmen, archers, and catapults vs. my swordsmen!!! maybe they updated it or something, but it works better now.