Or you get lucky, or possibly some airlines generally having good availability, e.g. CX. I had no flexibility at all for the booking mentioned above but managed to get the seats - availability around my preferred dates seemed good in any case. I suspect CX availability is pretty good on many routes, in fact. So much so that I ended up booking SIN-HKG-SYD because no-one else had a seat for SIN-SYD direct (not even Star Alliance).