It's all in the timing!
Let's take guest Joe Smith.
When he signed up, he used id joe.smith@whatever.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.
The account switches from joe.smith@whatever.com to deactivated-joe.smith@whatever.com
Joe is now free to again sign up with the same joe.smith@whatever.com user id since it is technically no longer in our system -- His second sign up was on May 7th at 10 pm at night. 