Basically, when you get in combat, you have a chance of victory. The easiest way to show this is via example.
Lets say you have 2 horsemen, no promotions. They have a base 25% chance to withdraw from combat. As they both have a 50% chance to win (exact same unit), they have a 25% chance to withdraw if they loose. That means that it will show 50% chance of victory, 12.5% chance to withdraw. This shows how it is only possible for him to withdraw if he looses. Now, if the horseman took a flanking promotion, he would have a 50% chance to win, but a 22.5% chance to withdraw because he has a base 45% chance to withdraw. This shows that his 45% chance to withdraw comes only if he looses a round of combat. It also means that he has a 72.5% chance to survive the combat.
The only other things to note is (with the exception of loki - damn that vile puppet) no unit can get a 100% chance to withdraw from combat. Everyone else is capped at 90% chance. Lastly, withdraw chance, just like strength, is additive, which means that if you have a base 25% chance to withdraw and take flanking 1, which grants a 20% withdraw chance, he would then have a 45% chance to withdraw.
-Colin