Just curious if someone know what this "dark matter" is:

SFO/NRT - booked on Eco, upgrade waitlisted for Polaris. Asked for position in waitlist, didn't seem to make the cut, so upgraded to PE during checkin 24 hours before plane leaves. See myself on place #1 in waitlist. See 7 open seats in Polaris. Checking repeatedly during the 24 hours, and ca. 1 hour before takeoff seats start to vanish. Ultimately i wait boarding as the last person and finally i get notification - seems like i got the last Polaris seat.

But so.. what could this dark matter be that fills up these seats so late, and without showing up in the upgrade list - is there some other place i could look at to diagnose where it comes from ? I wanted to ask at the gate but they where so horrible busy and i didn't want to be left at the airport alone after i finally got the upgrade and they where about to close the door ;-)