Er, I still would like to have an answer to Serack's question.
There are a few answers:
1) The person posted as a guest and never had an account;
2) The person had an account and deleted it, thus leaving their posts for posterity in a new, anonymous status;
3) The person HAD an account, deleted it, then opened a new account under a new user name and then changed the nickname to their OLD username, thus making it appear that they had come back from the dead when in truth they have not.
The most common answer is #2.
If I missed anything then please add on!
Hope that helps!