That's why you've checked the option "No, I will enter prices inclusive of tax" and as this is a WC's global setting, affects to all the prices. Anyway, you can add the tax to that Purchase Order's product using the "Add tax" button, and the line total should reflect the purchase price you set to that product.
Purchase Orders uses the same functionality as WooCommerce orders. If you try to create a WC's order manually and to add a product the same way you did for the Purchase Order, you'll see that the prices added are tax excluding too (if you have the option mentioned above checked).
Hope this helps.