Username search not working

I have set Name Indexing based on username from the back end and done a purge and re-build. I have also set the name field (not username) as NOT searchable. Yet when I use the advanced search it gives me the options to search on real name but does not give me the option to search on username. (I also have Display name format set as Username and I do not display the real name on any part of the profile once a user is registered.)

I did a full name search for a user and it managed to find them from their real name which it should NOT be doing.