A clone copies the refs from the remote and stuffs them into a subdirectory named 'these are the refs that the remote has'.
A mirror copies the refs from the remote and puts them into its own top level - it replaces its own refs with those of the remote.
This means that when someone pulls from your mirror and stuffs the mirror's refs into thier subdirectory, they will get the same refs as were on the original. The result of fetching from an up-to-date mirror is the same as fetching directly from the initial repo.