Does anyone know why row 5 DEF are reserved as "premium" seats yet 5 ABC are available to anyone? Is there a solid wall on the port side of the plane but not on the starboard side? Or is the legroom the same but they keep ABC open in case someone has a baby to hang on the bulkhead?

I have two upcoming transcons on 738s and would like to reserve a bulkhead in case my upgrades don't clear. From what I've read I certainly don't want to be stuck in the back of one of these cattle cars!