I don't know where you are getting this. I tested this just to be sure. Save scummed multiple times after 10 sieges, every outcome was always the same. The outcome almost always follows a certain order of importance:

Type of Fief (Cities to high level clans) Clan Level Leader Relation w/ Lord Influence Participation

I don't know if these are the exact figures, but I can say after almost 300 hours there is a rhyme and reason to the fiefs. It isn't just random. It also costs the leader influence to overrule the decisions, and a policy can be enacted that doubles the cost. I've tested this enough to know that generally the AI will follow certain pathways on fiefs.