This incidence may occur for a couple of reasons. First, Speakers added through your agenda might not be registered through your Eventbrite account, but the profile you made for them in the agenda will show up in the attendee list.
Another reason is that some of your attendees might buy multiple tickets for people with the same email. When this happens, the Whova system only builds a profile for the person who did the registering. The other attendees can be added manually in the attendees section of your dashboard.
Eventbrite also syncs once a day, so you can click the manual sync button to import any new registrants.