I agree with David.
Also, it's possible that the flight might be sold out with Delta still being willing to sell more seats. That happens for two reasons.
The first is that the airline can oversell the number of seats available because it knows that not everyone with a reservation will show up for the flight. Some will cancel. Some will oversleep or get caught in traffic. Some will miss connections from another flight. Some will reschedule for another day.
The second reason is that Delta will sell those last seats at full coach fares, significantly more than some of the other seats that were sold. If too many people show up on flight day, Delta simply offers a few hundred bucks to get a volunteer or two to take the next flight. Meanwhile Delta has the extra revenue from selling those extra seats at full fare.
Delta's not alone. All airlines, except for one or two of the low-cost airlines, operate that way.