You have an event registration form with the 'see who's coming' feature enabled. You may see attendees listed in the event record of Raiser's Edge that are missing from the attendee list on your NetCommunity site.
The event attendee list in NetCommunity will only display the participants that have opted in to be displayed on the list. If the attendee list opt-in attribute is missing from the participant's record, they will not be included on the attendee list in NetCommunity.
Per the article cited: Note: Within the Directory Part, for Records to Include, you may leave it defaulted to All Constituents. The Records to Include does not change which records are displayed in the Attendee List. The Attendee List results are controlled by the Participant Attribute added to the participant records within the Event in The Raiser's Edge.