gnome-simple-greeterwhich is part of the
gdmpackage that manages the GNOME login screen display the users ordered by how frequently they log in to the system. This can be quite confusing as it is generally different from the obvious sorting criteria like user name or numeric user id.
It took a little while of digging around to find out how the login frequency is calculated. As it turns out it is a feature of ConsoleKit which can be accessed from the command line as
In order to reset the count, it suffices to delete the file
$ ck-history --frequent
/var/log/ConsoleKit/history. By editing that file the login count can also be modified at will.