They are rare, but they do exist. Mostly due to older implementations of DNS registration. Most registrars automatically register the "non-www" version, as well.
I registered my site just yesterday, and while the prefix-less option was set by default, I COULD have removed it, causing this issue. Now, that would have been stupid (unless I fixed it on the server side, which had set up anyway), but it was an option if I cared.