How can Wordpress users not have an email?

I have been clearing out users that may have been created with automated structures and have found a number of users with no email address. I was under the impression that it is impossible for an outside user to create an account without a uniquely associated email address. How do you suppose this is being done? Is there some back door to Wordpress I need to know about that a non admin could exploit to create users?

The only ways that people can create an account is to either register for the site and get mailed a password or use the Ultimate Facebook plugin to login with their Facebook account. Under both conditions, an email address is logged.

What could be causing these users I have with no email address?