Missing entries in sources.list after fresh installation of Xubuntu

On my laptop I have Xubuntu 20.04. I've noticed that running # apt update && apt upgrade -y hardly ever results in updates being applied.

Upon doing a comparison of /etc/apt/sources.list between my laptop and a fresh Xubuntu (installed in a VM using the 20.04.1 point release ISO), I've noticed that the following lines are missing (as opposed to commented out) on my laptop's version of the file:

deb http://gb.archive.ubuntu.com/ubuntu/ focal-updates main restricted
deb http://gb.archive.ubuntu.com/ubuntu/ focal-updates universe
deb http://gb.archive.ubuntu.com/ubuntu/ focal-updates multiverse
deb http://gb.archive.ubuntu.com/ubuntu/ focal-backports main restricted universe multiverse

What could have caused this difference between the two installations and how can I avoid it happening in future?

Edit

1. It has been proposed to close this question on the basis that this is a duplicate. That question asks how to restore a broken sources.list file. I already know how to do that. My question is asking why the lines are missing the in first place. The two questions are not duplicates of each other. Whilst I am grateful for efforts to locate a duplicate, please be careful to make sure that it actually is before submitting a close-vote.
2. It was suggested in the comments that I must have edited the file myself. This is not the case. The file sources.list.save is missing exactly the same lines and has a modified date of 26 April 2020 which is the date that I installed the system (a few days after 20.04 was released in case that is significant).