Titles basically are hereditary in PKH through your male line and apply to your faction leader. None of this walk into a province and you suddenly become the local Lord etc.
Providing your new faction leader is of your true family then you can receive his titles. However, as applied then, you must pay your feudal relief to the King in order to receive your titles. Naturally you can only do this when its convenient to the King and clearly not if he's annoyed with you. Once you have paid your feudal relief you can of course then gain the benefits of tax revenue associated with your accepted title.
This is one of the little scripts that has been set up in PKH to add that extra medieval flavour... and thats just one of the reasons why its good to be the King.. and you can't play the King