IPB allows you to enable multiple login methods, and you can decide on a per-login-method basis if you want to use username or email address. Right now we have the internal/default login method, which uses username, we have converge which uses email, and we have openid all enabled.
Because internal uses username and converge uses email, the form has to allow you to enter both. On MOST forum installations, you'll have one method enabled, and only be asked for the data for that specific method (username OR email).
If you are a long-time customer, you would have a proper "username". That is, if you registered before we applied Converge to our site, or if you register here directly. If you first purchased the package through the client area, however, your username is actually your email address behind the scenes.
I know it can be a little confusing, but it's mainly due to our specific setup, and you wouldn't normally experience this confusion on your own site. :)