The answer here is almost certainly that a “chargeback” has occurred with a ticket purchased to your event.
What is a chargeback? A chargeback occurs when a ticket purchaser goes to his or her credit card company and “disputes” a charge found on his or billing statement. Saying essentially, “I did not make this purchase,” or “I did not intend to make this purchase.”
Chargebacks are rare, but when they do happen, the money for the ticket is automatically removed from our system and the ticket is cancelled. Presently, our system does not differentiate a “refunded” ticket from one that is subject to a “chargeback.” It recognizes only that the ticket is no longer active, and therefore marks it (and the corresponding dollar amount) as “refunded”… We are working to update this functionality. Hopefully the system will clearly differentiate the two soon!