I see, I thought I explained decently. Battles are basically like pulling straws. You get as many "straws" (aka chances to win) as you have mercenaries, and the enemy gets as many "straws" as they have mercenaries. So if you have 8 mercs and the enemy has 2 mercs, you have an 8 out of 10 chance to win.
English is not my first language, so I understand if I didn't explain it well enough. How should I have explained it in the game instead?