Windows UNC paths with parent_path()

Presumably, calling parent_path() repeatedly (starting with all valid paths - ie those returned by real_path()) should return valid paths? Ex:
IE - all of the above are valid paths.

If so, then using a Windows UNC path doesn’t seem to work right:
This last return value isn’t a valid path.

Isn’t UNC path \server\share more like unix “/” and the parent should be “”??