Problem with apt-get update on Ubuntu 20.04 lts

The reason this happens is because not all the mirrors get updated at the same time. For a variety of reasons, updates get rolled out on a schedule and can take a few days for all the mirrors to be in sync.

If you see this problem, the best solution is to just take a nap or wait until the following day. If you'd like confirmation, or to see when your mirrors are updated, you can actually just look here. That will show you the status of mirrors, including how far behind they may be in the sync process.

You can also change your mirrors to new ones, which might be a good idea if it's a major security issue that you're updating to fix.

If you decide to change your mirrors, open 'Software & Updates' from your 'Preferences' and the very first tab has a drop down menu labeled 'Download From:' and pick alternative mirrors. If you're curious, it's generally considered best practice to pick the mirrors that are geologically closest to you.