In D1, sometimes seats are blocked because they *might* be needed for crew rest or because they're set aside for possible use by handicapped passengers. Sometimes you can call DL and get a seat assignment, subject to change if there's a handicapped passenger that needs the seat.

IIRC if you ask the airline, there's a DOT regulation saying that they're required to tell you (honestly) whether the flight is overbooked in your cabin class if you ask specifically.