The following apt repository is always configured in images provided by Olimex :
deb http://repository.olimex.com buster main
But, depending on the image version used on installation, another repository is sometimes configured in /etc/apt/sources.list.d :
deb http://staging.olimex.com/staging buster main
I recently discovered this because the kernel version was different, and had a different behavior with up-to-date A64 boards.
For example, the latest kernel of repository.olimex.com is a 5.8.18, but is 5.10.4 on staging.olimex.com/staging. This 5.10.4 seems to work, but does not enable all the board temperature sensors that are working in 5.8.18, at least.
The latest image A64-OLinuXino-buster-minimal-20201217-194545.img does not have this staging repository, but some previous ones had it. At least :
This is a problem because the boards installed with one of these versions might grab unstable kernel versions. It might be expected for testing these kernels, but probably not for production.