Their willingness to clear an upgrade is directly related (inversely) to their belief that they can sell the seat for cash. That varies significantly by route, but also by how far in advance the flight is. And they have lots of historical data on ticket purchase patterns by which to gauge that likelihood. Presumably they already found 4 people willing to buy the seat to SYD, and today, they think they will find more. If they don’t, they’ll open the seat(s) for rewards and upgrades.