I've been on DCA-ATL M88 with empty seats in F. Those UG lists are long (25-60 not uncommon). For some reason, me and my friend were cleared many days out (only PMs) so I asked the FA as the door was closed what was going on. She was equally shocked as I was.
PS, some non-revs, jumpseators, etc are out of pocket PMs too, just sayin.