Thanks. I KNEW I should have asked here first. Why does Delta hold back some of the seats for airport assignment?
The irony is that we had zone 1 boarding our flight from Atlanta to Phx, but DH spent so much time getting takeout at the food court that we were among the last to board.

By the time we finally boarded, there was no more room for our carry-on bag so it had to be gate-checked. I had both boarding passes or I would've been tempted to go ahead without him.