Events - Registration Form Setup

I recently set-up an event and created an contact list for those who registered. I put a check in both the boxes - 1) limit to one registration per email address and 2) allow registrants to opt in to marketing communications. My expectation was that this list would contain only those who completed the registration form; however, the list also added people who went into the event and did NOT register. Why would people who did not complete the registration be added to this list - did I mark something incorrectly?