Impossible FBA 'Inventory Reimbursement'

We had an FBA 'return' a little while ago where - as far as we can tell from the return details in seller central - the items were never actually returned. At any rate, Amazon have reimbursed us with an 'inventory reimbursement.' My understanding is that this means they have replaced the actual items rather than giving us the cash. To be clear the units are in the 'Quantity Reimbused [Inventory]' column on the report. (NOT the 'Quantity Reimbursed [Cash]' column.) The relevant stock movements are shown as 'replacement inventory found' in the inventory ledger. There is no corresponding record of the customer return being completed.

However, Amazon have no possible source for this inventory. It is an own-branded product of ours, we are the only supplier and they have not bought any replacement units from us. So what an earth is going on here? What have Amazon replaced this inventory with? Or is my understanding of the meaning of 'Quantity Reimbused [Inventory]' just wrong?

(We're not new to Amazon, but we are new to FBA. It's been going OK, so far, but this particular incident is a bit confusing.)