Dan,
In the past there has been a difference between the Campaign data and the Email Performance data, but in this situation, it seems very different. I would speak with Support.
It is possible that you are looking at data that is out of sync, that is, it is showing you a complilation of data and a process on Marketo's side hasn't completed somewhere, thus two reports are wildly out of sync. It's also possible that you ran the report early - I usually wait 72 hours before the email data is considered "complete" enough.