I've been monitoring availability on SQ flights LON-SIN and now SIN-LON. I've noticed that seats seem to get released at various points close to departure.

When I booked LON-SIN in early July there was no availability for weeks ahead, however, 48 hours before the flight we took, four seats were made available.

I'm now trying to alter the return flight, currently ticketed on SQ318 2nd September to SQ322 1st September.

I've been checking for a few days now, and it seems that there is availability from 3rd September onwards, but little or nothing before then. HOWEVER, it seems that most of the time there is a seat or two made available about 48 hours before the flight. Not on all of the three daily flights, but more likely one.

Where does this inventory come from? Are these cancellations? Seats from aggregators? Seats they haven't been able to sell to high status frequent flyers?

And there is now ONE seat on SQ322, on 1st September, as desired, which wasn't there yesterday. Unfortunately we need two seats so this isn't much use, much as I'd like to grab it.

Is there a pattern or process to these last minute seats?