reason why user is gone after few days, it's mostlikely, he did not verify his e-mail so then if he does not do it in time the account becomes deleted automaticly. But if he has done validation process then admin must delet this account.
I can confirm this, since i have done a test myself. I signed up on my own site and did not validate the e-mail and in like 1-2 days my account vanished from the database and site. Then site keept saying my login detail's are incorrect. In admin panel stats said etc 2 users with 2nd account but once it vanished it said 1 user.