Over the past year I've noticed that the term "full flight" is completely misused by flight attendants on WN. Yesterday the FA's said over the PA that the flight was "completely full" and that "every seat would be taken". We left with 16 empty seats. I guess they do this to encourage people to sit down in a middle seat rather than saunter all the way to the back of the plane looking for a window or aisle seat. Still, it's kind of annoying and I will often times choose a seat futher back if I think there's a shot that the middle seat will be empty (obviously there's no point if the flight is "completely full".) My completely anecdotal experience is the following:

Full Flight = up to 20-25% of the seats are empty
Completely(!) Full Flight = up to 20-25 seats are empty