What are the basis for exchange rate variance calculation on the invoice?

There are multiple foreign currency invoices showing unexpected behavior for calculation of exchange rate variance. Some invoices are showing exchange rate variance based on this formula {Invoice Qty x PO Price x (Invoice Exchange Rate - PO Exchange Rate)}. However, majority invoices are showing exchange rate variance based on this formula {Invoice Qty x PO Price x (Invoice Exchange Rate - Receipt Exchange Rate)}

Please guide us with the system behavior on calculation of exchange rate variance. Kindly guide when PO exchange rate is applied and when Receipt exchange rate is used for the exchange rate variance calculation on the invoice.