Even though we do a preliminary tax/shipping calculation in the cart before checkout, PayPal checkouts will not show shipping/tax amounts until the buyer has provided their actual shipping address during checkout, so PayPal can contact our system to make sure the shipping/tax charges are appropriate for their location.
This prevents confusing or misleading the buyer in case the address they provide does not match the location they'd specified in their cart before checkout, which may change the calculated charges. If the order does not require a shipping address, tax will not be recalculated at checkout, as PayPal does not inform us of the buyer's location in that case.