Questions? I've got answers!
This actually took some digging so bear with me.
In the default of vBulletin, there is a group that is defined as "Users Awaiting Email Confirmation". The default of this usergroup is set to "Registered User".
So, here's what likely happened:
1. User joins FlyerTalk. User is placed into "Users Awaiting Email Confirmation" usergroup and is given the "Registered User" title.
2. User confirms email and is automatically promoted to "Registered" usergroup, thus removing the "Registered User" title as they are no longer a member of the "Users Awaiting Email Confirmation" usergroup
3. User contributes some posts
4. User forgets password and uses the "Lost Password" feature to request a new one. When he/she does this, he/she is placed back into the "Users Awaiting Email Confirmation" usergroup and is reassigned the "Registered User" title. As such this title would now appear under the user's name on his/her posts. Once the user reconfirms the new password, and is put back into the "Registered" usergroup, the title would go away again.
But, that doesn't explain the issue with the member mentioned in post #4.
In this case, and a few others like this member, we think what might have happened is that, after requesting their new password, they wrote us an email saying they hadn't yet received it. In cases like this, we usually assign them a temporary password and move them to the Registered usergroup manually. We probably did this and neglected to update their User Title, which would have occurred automatically had they gone through the normal process. And voila, they are left with the weird "Registered User" title.
Looks like there were only seven such cases, and all have now been manually corrected (Thank you for keeping us on the top of things). We're going to keep an eye on this to see if any new ones pop up unexpectedly and we still aren't quite sure if this will completely resolve the issue.
We have removed the "Registered User" title from the "Users Awaiting Email Confirmation" usergroup. There are currently over 11K users in this group with zero posts who have the title, but should they register the title will be removed (and if they ask us to help them, we'll be careful to remove the title manually).
And now you know ... the rest of the story.
Truly, thanks for the question and it was a nice challenge for us to finally figure it out.