Join Date: Dec 2003
Location: Redondo Beach, CA USA
Programs: UA 1KMM, Bonvoy LTE+A, HH D, Nat'l EE, Hertz Plat, Avis PC
Posts: 3,713
Yes, some people don't choose their seats in advance (which is why the # of seats / # booked is the right indicator of available space, not the number of seats that show unassigned).
As for getting upgraded and not showing up on the upgrade list anymore, that happens and is nothing to worry about.