This seems strange as well.
These offers are for those that just missed out on an MBBS place. How can the school know who just missed out until they get the declines - eg LBoG today - and then make late offers?
Last year I know they were making phone calls a few days before semester started re guaranteed offers.
This is because they also offer to students who didn't make the ATAR of 95 but did manage to have a competitive enough UMAT score. Hence why they are able to give earlier guaranteed offers to students who gained the ATAR of >90 and a competitive UMAT well in advance.