I just did an experiment similiar to the one above. I had a general of peasants, as well as the enemy general of peasants, both never bothered two engage. I had 2 units of Mailed Knights and the enemy had 2 units of Armoured Spearment. Now when I walked them up to the enemy they began dying but if i clicked "move" on the ground behind them the cavalry just walked through the units completly decimating them. I think the problem lies in that fact that infantry don't really have any staying power and cavalry just walk through them like they're not even there. The AI seems to do this. I remember in a siege having 8 units of spear militia almost completly decimated before being able to finally destroy 1 unit of jinettes and 2 units of mailed knights. The mailed knights just kept walking through my men killing them in massive numbers. I finally called in my general and he dispatched them but I lost a lot of spears.