apt - What causes "Failed to fetch ... File has unexpected size. Mirror sync progress?" error?

I've seen this error pop up for different people for different repositories. I was just curious what is the exact this error, as I got it the other day. I think I ran "sudo apt update."

E: Failed to fetch https://download.docker.com/linux/ubuntu/dists/bionic/stable/binary-amd64/Packages.bz2 File has unexpected size (9051 != 9594). Mirror sync in progress?

From my impression, it is something that happens when the package containers mess up with something.