Since you mention this is a "later" flight, is it possible DL was holding back seats to protect them for potential misconnects, so pax wouldn't end up stranded overnight? That's my best guess to explain the phenomenon described by the OP, although I don't believe he mentioned the specific routing.

I fly AA more than DL and have noticed AA doing something similar on later flights for routes like ORD-STL or LAX-SAN (last flight of the night from a hub to another domestic airport, where you may very well have high paying premium passengers connecting from overseas who need to make the connection to be able to get home or to work the next morning).

It may be more of a mess and lead to more re-accommodation expense and loss of passenger loyalty to DL if they were forced to accommodate high value connecting passengers overnight, as opposed to perhaps the potentially smaller amount of revenue they could gain from selling those seats as TOD upgrades or even day of departure F purchases.

In any case, I doubt the situation encountered by the OP was a fluke. I would think it's much less likely that this is a DL mistake/fluke and much more likely that the inventory management here was the result of perhaps quite intricate research-driven rationale.