VAT calculation on PO Total

 When I add tax code in Purchase order shipment , Purchase order total in the header level does not changes. For instance, I created a purchase order having one line ( unit price 100 USD and number of unit is 1 , it means line total is 100 USD). Total value in the Purchase order is 100 USD. Now I added a tax code (VAT ) having 15% in the shipment line of the Purchase order and saved the Purchase order. I found that total encumbered amount is 115 USD but the Purchase order total is 100 USD.

Is there any way that I can see the purchase order total is 115 USD?