They hold back a few for last minute sales. I've seen as few as 2 held back and as many as 5 in recent weeks. When you check in, you will see where you are in the hierarchy (but the hierarchy changes as people check in, so you may be #1 23 hours and 59 minutes before departure and then #56 at departure), and then if nobody buys those seats and you're high up enough in the list when you scan your boarding pass it will print your new upgraded seat assignment (in my experience the app will update once boarding begins usually too, though it will still say Comfort+ it will show your correect upgraded seat number).