When you configure the cart to use the ship calculator the cart calls the ups web service api with the ship address entered by the user and the ups web service returns a result with ship cost, its a straight pass thru to ups the cart doesnt calculate the rate. I recommend going to the ups website and entering the shipping address for the order in question and see if it returns a ship cost. The address may be a po box or and address where ups doesnt ship to or it could be a data entry error by the user. Also, what version of the cart are you running?
-Scott