There are 5 seats in coach and 7 in F for a total of 12 seats for sale on the plane.
That is incorrect.
DL is overselling Y, and part of the logic for this is based on upgrades and op-ups.
If somebody bought all 7 F seats, then they now know Y will not get the upgrades so they will reduce Y inventory.
It is also possible there are more than 7 F seats available. It would not stun me if JFK/LAX held a few back in order to score the very expensive last second F (BE) seats.
It's a complicated process, and nobody outside DL will know exactly how it works (though I am certain many know the general concepts).