I don't have a specific answer for you, but I DID notice that if you enable the option for "if someone registers with an email address that already exists, update the existing record, don't create a new one", then it will create a new Private ID each time the user tries to update an existing record. Not sure if you have that turned on or not.
I turned it off because I needed to allow multiple users with the same email address, but otherwise would have left it on.
Some additional work needs to be done in this area. It should notify the user that they are updating a record, and ask if they want to do that. Or something of that kind. It kind of threw a wrench in my database when users updated their information using the same email address, and all of a sudden the old Private ID was invalid (and so was their original email with the link). Quite confusing for some users.