Just to reiterate what went on here:
Most of the issues are from things like people who have merged their accounts in the past, people having multiple accounts with the same username and not realizing it, or recreating an account because they no longer had access to the email address they initially used. In cases like these, you won't have to change your username, I just need to double check everything, which gets time consuming.
If you think this may be the case, send an email to email@example.com with the title "Nickname" with any email addresses you may have used in the past, as well as your full name. I'll check out the situation and get back to you as soon as I can.