As I have posted previously I had one wonderful occasion when a J seat became available the day before I wanted to fly, got to the airport and found the flight fairly heavily overbooked in J, so got an op-u/g to F.
Just this week I was looking to advance a trip and get onto a flight that was fully booked in both CE and ET. Then 2 days before departure, 2 ET seats opened up, one of which was available as a standard (not just Gold) Avios seat, Obviously the flight was 100% full on the day..
No rhyme, reason, logic or anything else to why the computer says what it does !