They most likely all have a matching fare structure. Given the size of the RJ F cabin, there may only be one or two discounted seats (or none until seats don't sell). However, AA does not want to release very many discounted F seats on a hop like that because they can lose a long haul international premium pax over it. If they sell you that hop at say, the $797 fare, they can't sell an F or J ticket SAV/NRT without the RJ segment being in Y. High value flyer will go to another airline.