The pricing conecpt is built in SAP in such a way that there can only be one price in the sales document. So whenever there are multiple condition types (of condition class B Prices) in the sales document then by default system considers only the last condition type as active and the previous condition types will be made inactive. Since the conditions are inactive they would not add up to the netvalue of the document.
This does not apply for other condition types with condition class other than B (prices). Hence, all the discounts are considered in the net value of the document.