you may see a shipping quote on the cart page if you have already entered your address, then revisited the cart page.
a rate shown on the cart page may not be accurate. the shipping gets calculated when the checkout page passes to the confirm page, then gets stored in a session.
if you continue shopping after the initial shipping rate is calculated, it wont be recalculated until you go to the checkout page and resubmit the shipping details.
dont be worried by the shipping shown on the cart page, it will get recalculated when you go back through the checkout page to the confirm page.