It has to do with seats sold versus seats unassigned. The flight is likely indeed oversold (as in more tickets have been sold than there are seats on the flight), however there are still unassigned seats.

It doesn't have anything to do with being held for nonrevs. Seats are not held for nonrevs. They are blocked for gate assignment for various reasons (passengers with disabilities, families, et cetera).

So in actuality, both the agent and the list are correct. The flight is oversold, and there are also unassigned seats.

Edit to add: Blocked seats will not show available on DL.com