The IT in question shows the number of available seats on the upgrade list as the number of currently unassigned seats, not how many seats are actually available which is mildly annoying. If you look at the standby list and see First next to someone's name, then that's a paid F pax that didn't have a seat assignment. I've noticed a common theme that if it says Main next to their name but an F seat assignment then something fishy might have occurred.