[Niarg]

I've had this happen as well, although not with the disparity you have.

Twice I've had my army try to invade a province and twice I've been pushed back and both times I won the battle. However that was largely due to spells as I was outnumbered and outclassed in troops both times so it may be largey random with people just not noticing when the huge army attacks and pushes back the smaller one.

Another worrying thought I had was that movement order may be determined by unit ID, so certain commanders will keep being pushed back due to an attribute you can't see...