It's all in the timing!
Let's take guest Joe Smith.
When he signed up, he used id email@example.com
His first sign-up was on May 5th. For whatever reason, the account was deactivated by an administrato on May 7th, at 12:00 PM.
Joe is now free to again sign up with the same firstname.lastname@example.org user id since it is technically no longer in our system -- His second sign up was on May 7th at 10 pm at night.