This is because, in addition to the cost/unit amounts for the items that have sold, the COGS line on your Profit & Loss report includes costs captured from your reimbursements, returns, and removals/disposals/liquidations for the timeframe as well.
On the Accounting>Reimbursements page, you will see cost/unit accounted for on transactions with the descriptions like Lost_Inbound and Damaged_Warehouse where the reimbursements were in cash. These amounts would be included in the COGS line on your P&L on the date of the reimbursements.
On the Accounting>Refunds page, refunds that are unsellable, have a disposition reason, and were returned back to your inventory will receive a cost/unit credit to match Amazon returning an item to your inventory. These amounts would be accounted for in the COGS line on your P&L on the date of the refunds, and the cost/unit credit ensures the cost is not counted twice as an expense to you if the item sells again.
Removals, Disposals & Liquidations
If you remove an item, you can mark it as unsellable on the Accounting>Disposition Management page. In this case, the cost is tracked as a debit to the COGS line on the date the removal order was placed.
The Disposition Management page is also where costs are captured for disposals, liquidations, and MFN refunds. The cost/unit amounts from these transactions are tacked in the COGS line for the dates they took place, as well.