Why is some content shown as being posted by an Unregistered User?

Any content submitted by registered users is posted directly to the site, and their name will be shown.

However, unregistered users can still submit content. In this case it is not posted automatically to the site. Instead, it is mailed to the Site Admin, who manually checks the content and adds it to the site. In these cases the content is shown as being submitted by Unregistered User.