If a customer changes the quantity of the product in the shopping cart and clicks 'Checkout' instead of 'Update Cart' and then 'Checkout', the customer may not realise the quantity has not been updated and proceed to purchase the unmodified quantity of a product.
It would be better if 'Update Cart' logic was applied when 'Checkout' is applied similar to the way clicking 'OK' in a Windows application automatically does the work that the 'Apply' button does.