The problem is that it's listing all of the users in your site collection that have a username containing the membership provider. When you change databases, it doesn't remove the SharePoint users from the site collection. In order to remove them from the
list, you'll have to find all of these "orphaned" users and delete them from the site collection (People and Groups -> All People).
In the next release i'll fix the User Management screen to only list users that actually reside in the membership database.