How discount amount is calculated using B2B Peppol ?

When having a payment term using a lines only discount, the calculation within Peppol is different then in screen when applying receipt. Assume the following scenario. A line of 100 euro's excl VAT and VAT 21%. A discount of 0.4% is applied based on lines only. So the discount should be 0.4 euro (100 * 0.4% = 0.4). however when sending and creating the peppol message a discount of 0.48 is calculated = 100 * 1.21 *0.4% = 0.484 euro. Why is this done. the discount setup is without vat, so why is the calculation incl vat?

So the peppol message should behave as a financial discount. when paid earlier then 0.4 euro can be deducted otherwise payment will have to be executed on full.