Default users removed from the Default User Role may not seem to be Available Users in some Group Roles otherwise created. Assign an Available User to a role, you may need that they still be 'available' in case you need to assign them to another role as well. So at what point do they leave that Available User column? How can a user show up as 'available' to one Role and not to another?
When importing users you should notice an <ownerRoleName> tag. The owning role helps explain for which roles a user might show as 'Available User'. A brand new user that has not been predefined should automatically go in the 'Default User' Role. What is the significance of the ownerRoleName?