Those users are accessible by name. When passing a numeric ID, the user does not have to exist in the container.
So basically, if you pass:
docker run -u myuser:myuser busbox
myuser doesn't exist in the docker container so it has to be created inside the docker container.
But if you instead pass the
gid the username only has to exist in the host system.
docker run -u 1000:1000 busbox