A common trick the universities use is to tell candidates that they'll upgrade them from a conditional (e.g. AAB offer) to an unconditional offer if they make that university their firm choice within X days.
I believe it's a way for them to better predict how many students they'll have next year, otherwise it can fluctuate pretty wildly based on exam day results.
Edit:
"38% of applicants (97,045 applicants) received at least one offer with an unconditional component in 2019, increasing from 34% (87,540 applicants) in 2018, and continuing the year-on-year growth in offers with an unconditional component since 2013." [1]
This year at least one Oxford College has already decided to honour all offers without looking at actual A-Level results , whereby effectively turning all offers into unconditional offers [1]. Granted, this is not a normal year.
Let's be honest, though, if you got an offer from Oxbridge you're very good so they're not exactly taking a risk and it gets them brownie points.
[1] (PDF) https://www.ucas.com/file/250931/download?token=R8Nn7uoI#:~:....
[2] https://www.theguardian.com/education/2020/aug/15/a-levels-r...