[quote=@Darkspleen]
I assumed as much.
[/quote]

There's always that chance of inheritance. Now that I mention it, if your family from the duchy inherits some land from outside you can choose between having one member leaving the duchy and being a noble somewhere else or you can incorporate your gains into the duchy. But if the land inherited is significant there's a good chance some of it will be taken away and given to other nobles (sharing is caring), as opposed to having some financial and military support at times of war in case you choose the former. You should carefully wager, though.