Something I noticed in autresolved naval conflicts. And may also apply to army autoresolved battles.

The second unit in your stack appears to always be the key unit. So if your second unit is the strongest unit in your stack you will more than likely win. But if it is your weakest you are less likely to win but if you do win it appears the casualties will be higher for all units.

And quite often in naval battles it appears that your second unit does most of the work all the time. As in gets the most casualties but also inflicts the most.