Memberships Inconsistent + Roles

You describe the default membership as: 'Content is available to all logged-in users who have not yet joined your site as a member.'

Please explain how someone could be logged in but not yet joined the site as a member.......

Is there a way for me give users a choice to pick a role when signing up?
Is there a way to ensure that users are directed to the 'success' page after registering? Mine appears to be broken and now displays a blank page instead.