This is pretty common with Delta. Often there will be seats availables that will not show up on the seat map because they are being held for gate control. If there are open seats in first then coach passengers with status will likely be upgraded and that will open up additional coach seats.

Delta will assign you a seat at check-in. You may be able to negotiate a better seat at the gate. Depending on the length of the flight the C+ seats are a nice improvement over standard coach.