My wild guess why they are processing late-2020/early-2021, very fast is:
May be IRCC-Sydney might have automated their systems to scan and upload the full files instead of sending physical files to local offices. So IRCC working fast on new-online applications.
Since all 2019/Early-2020 applications are paper based, which have already been sent to local office, and there is no way for them to access the physical files (and) no one is going to local offices, so there is no chance to either scan them into system (or) to work on them.
This might be the reason that the lucky batch of late-2020/early-2021 getting fast invites and processing ahead of 2019. So all 2018/2019 backlog will be cleared once the physical offices opens up.
In any case, it is the worst bad luck in life.
Last week I read a news that, The US passports office had around 2 million pending passports to print by end of 2020, so they took it as mission critical and completed all 2 millions in just 1 month by spending extra money. That is what I call respect for law, professionalism and accountability.
But here in Canada, we are at mercy of their emotions instead of rules and policies. Very unfortunate.