Hi,
It is because the shopping cart page doesn't have any itemID and the "itemId" of the page you are currently visiting is being carried along.
Create a joomla menu item for "Virtuemart Shopping Cart". Then everything will be much more organized. The link for "show cart" comes from the Virtuemart Component.
For better diagnose, disable the SEF URL and see what itemID the "show cart" link is getting.
Regards.