Hello Caiti,
OnArrival app will always show you the attendees who are currently in 'Accepted' status of the event. If anyone registers and later cancels their registration, they will not appear in the OnArrival app. So you can search the event's attendee list on the back-end and see if these people are still registered or in cancelled status. You can also go to the attendee record and click on 'View History' under the Details tab to see if when they registered and cancelled(if cancelled). I hope this helps for your future events.
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Debanjana Guha
Senior Onboarding Specialist
The Princess BrideUnited States
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