You likely did not select all the usergroups available when configuring "Usergroups" parameter in your userlist. It is absolute in that it will only display users that have one of the selected usergroups. For super users you need to select the "Super Users" usergroup in addition to "Registered".
On a side (but associated) note. Is there a way to prevent specific users from displaying in the userlist?
Create a checkbox field then condition against that checkbox field in your userlists filter settings.
krileon
