That is the normal behaviour of the module as it tries to finalize the payment at PayPal during the Express Checkout flow. If the customer goes through with the sale, when automatically returning back to the store, the store will detect that a different total has been calculated and the customer is presented the checkout confirmation page which they must confirm to finalize the sale.
This will be confusing for the customer as the last PayPal button presented is "Pay Now" (to try and finalize the order here). This works wonderfully if Instant Update is enabled and is working on your server (eg, it requires a HTTPS/SSL certificate).
The next update to the module changes the behaviour to finalize the order when returning back to the store (the last PayPal button shown is "Continue", not "Pay Now"). This allows the customer to review their order on the stores checkout confirmation page to finalize.
Here is more information regarding the change of behaviour:
http://forums.oscommerce.com/topic/397563-paypal-express-checkout-flow/