Invoice Price and Quantity Variances

I have a query about how payables calculates the price and quantity variances when the standard process is not followed. I know that there are process issues in the example I give below which I can fix but I'm not sure that oracle is behaving as expected.

I have a purchase order with 4 lines and I receive 3 invoices before the receipt is created. Each invoice is matched to the PO and a line and the holds that are created during validation are manually released to allow payment.

Item 1

Ordered 100 @ 42                  billed 120 @ 37.8

Qty Variance (120 - 100) x 42 = 840        Price Variance 120 x (37.8 - 42) = -504

Item 2