It appears that the ai gets a huge advantage, even set to corporal. When I set the game to play the ai (highest setting was Lt. Col), the ai was scoring huge casualties consistently, even on corporal, no matter which side I played, while the casualties I inflicted were more towards the norm. Not knowing exactly what each difficulty level means in terms of game play and ai advantage, I'm not sure how to gauge the results.
Just taking a stab in the dark - my favorite kind of stab - I wonder if maybe your guys were set to "Advance" instead of "Assault"? That, or you didn't have skirmishers out? Also, did you have more than about 350 guys in your unit? That triggers extra casualties.
I find that I can usually keep casualties even or better than my opponents unless I either screw up my stances, are in a very exposed location, or start taking more fire than I'm giving. It's when I have someone with an exposed flank, or I forget to change from Advance as I close, that I start stacking up bodies.