Different factions have different AI settings, I think. It probably affects their diplomacy personalities. The seleukids were, the other day, repeatedly besieging my towns. I'd immediately propose a ceasefire while my town was still surrounded, along with a demand for 1000 mnai. They'd accept and give me the cash, then seige my town again... repeat. I really don't know why they kept attacking me, and then ceasefiring with a 1,000 bonus, without a single battle. Probably the strangest thing I've ever seen the AI do.