If they are TOD upgrades offered during the check-in flow, these show up as "cleared" upgrades though, right? That's certainly been my experience.

I see a lot of this fluctuation in availability as well on most long-haul flights inside the last 24 hours before departure time. I've put it down to people on flexible tickets moving between flights, corporate travel agents protecting pax on flights without ticketing them, staff travel, etc. Very often the "booked" number will increase without any seat being selected.