It's funny, but these forums show signs of having many sock puppets (alternate user names). Some of them even come in and validate their own previous postings, but you can tell.
I used to operate a huge political forum cluster and it was common. The foolish ones would use the same computer (unique IP address) for multiple accounts, until we put a stop to that. Then we began to see the same person using different IP addresses and attempting to create new accounts from them. And on and on.
In some cases a person will need to re-join if they lost track of things; people now days have many email addresses online. One sports forum I know of will not allow membership without a phone call!
But, heck, it makes for great forum population figures.