72 hours prior to the flight, QR blocks as many seat as passengers bought a ticket without selecting a seat (these seats appear to be blocked on ExpertFlyer but are selectable at Online Check in), not counting the seats next to a status passenger and front rows and other special seats which were already blocked before that time (and might not be selectable at Online check in). That can give you an idea of the flight load (and chances of free upgrades for Y passengers) but not a precise idea as these special seats (front rows etc) are blocked without always corresponding to a real passenger.

But I don't know why they block the seat next to you when you move your seat selection, as you are not a status passenger. I guess it's due to the fact there are only 2 unblocked seats on the J map and they prefer to keep the 2 unblocked seats together in case they need them to relocate a couple at check in or boarding time as MNL flights are often filled up to the maximum with QR Filipino employees in the last minute.