Is that something I have changed in error? Or is it by default?
This is something you set. The name you see at the beginning of the command prompt line is what you have named your Pi. Then you later changed "localhost" in your hosts file to the same name
An example below: the device is named Pi-3B as shown on the beginning of the first line, and the mapping to that name is shown in the local.list file.
[pi@Pi-3B:~ $ cat /etc/pihole/local.list
192.168.0.100 Pi-3B
192.168.0.100 pi.hole