Under what circumstances may an NPC faction capture a town?

So the other day, for the first time ever in vanilla M&B, I saw one NPC faction succeed in capturing a town from another. To be precise, the Khergits took Uxkhal from Swadia, while I was off working for the Nords and paying the situation no mind.

Later I rode by just as a huge group of Swadian lords were attempting to take it back. The Swadians had 900 men while the Khergit garrison of Uxkhal only had 300, and yet the Khergits won handily.

Clearly there is some sort of large bonus given to the defenders when two factions contest a town without player involvement. This would explain why they don't take towns more often. Except later on the Swadians DID succeed in taking back Uxkhal, after the Vaegirs declared war against the Khergits and presumably helped weaken them.

My question is, are the dynamics behind this clearly understood or documented anywhere? Exactly what rules are in place to keep one faction from wiping another out, and how flexible are they?

the NPC factions usually don't manage to wipe one another out because the remaining lords will be concentrated on a much smaller location. I won't claim to understand the battle calculator but as far as I get it, yes defenders in a castle get a big bonus. In the native setting horse archers also get a big bonus, which is why the khergits are always doing so well (I never let any battle with khergit auto resolve if I can help it, going out of my way to help others) - that plus they are usually fast enough to catch enemy lords means they can rapidly destroy an enemy faction while their enemies have a hard time catching them.

Thanks! There's supposedly some other rules in place to keep factions from being wiped out too, but no one seems to know exactly what they are.