Regardless of the stated odds, lurking somewhere is a "fudge factor" giving the following effects :-
1. If it is utterly essential that you, the human player, win a battle, you will lose.
2. Far more often than the laws of chance suggest, a much weaker defender will land a string of five or more successive hits on your unit, and thus kill it. This is particularly the case if your unit is led by a Warlord.
An optimist is a pessimist with no experience, remember !