another thing as told to me by a Delta gate agent is what class you list for as a non-rev matters as well.
if you list for first/business, and there are only seats in coach, then they'll "downgrade" the listing to the coach seat.
however, if you list in coach and there are none available, but first/business is open, the system will not automatically "upgrade" your listing. you'll be left behind. if the gate agent is busy they may not have the chance to change it in the system and help you out.