Incomplete Registrations are pretty common, especially for paid events. Potential attendees will complete most of the checkout process and then abandon it. For paid or ticketed events, it could be as simple as them not having their credit card on hand.
To clean up your Attendee List, you can delete these registrations at any time. To delete or edit an attendee, go to your Attendee List, click on the name of an attendee, then click on “View/Manage”. The Attendee's info will appear. To delete, click on “…” and select “Delete Attendee.”
Someone with an Incomplete Registrations can go back to register at any time. This will create another entry for them in your Attendee List.