The Pope calls a crusade for Sicily, I (King of Castille) join in, and we win. Mr Pontiff grants me the title of King of Sicily, and I get the island plus Malta. A few years later I usurp the Duke title, and give it to one of my relatives. Now, Castille is Agnatic-Cognatic Elective, but for some reason the Ducy of Sicily is now Agnatic-Gavelkind. Why is that? Shouldn't it have the same succession laws as Castille, or at least Sicily (which is also Agnatic-Cognatic Elective)? How would I change things from gavelkind?