If you open the product from the link in your royalty history report, it will show you the product as it was purchased by the customer. I don’t mean their design, you can’t see that, but you can see what size/paper/cut they chose. That might give you a clue.
Also, are you sure it wasn’t a transfer to a completely different product?
And there’s always the possibility that they re-designed it completely. I’ve had customers post pictures in my reviews, and sometimes the only part of the original design that they kept was the artwork!
Hmmm. I wonder if the customer had some kind of voucher or Zazzle credit? 🤔 I’ve noticed discrepancies like this before too, and always assumed they must have had some kind of a discount that was automatically applied, but by all means do investigate it with Z!