I am guessing the seats are set aside for crew rest. I have had routes where for whatever reason there are two seats held for crew rest and the captain gets to decide which seat they want other gets assigned to a passenger. I have also been told that if delta one has empty seats the captain can opt to have the seat behind the crew rest remain empty. I can’t tell you how many times on a last minute business trip I have only been able to get a seat in the middle section but by asking the gate agent have been able to get an aisle side seat that was being held for crew. Now the planes have more crew cabins built in but I still run into saved seats that can normally be assigned at the gate.