Hi,
it must be related to cache (either the cache of Joomla or your hosting server's cache) or your browser's cache (temporary internet files) cant be sure.
Try this;
Disable our module from your website and publish the original "VM Shopping Cart Module", (the one that comes with the Virtuemart-AIO).
If you see the same behavior with the official shopping cart module, then it means something is wrong with your website or the cache settings. If the other way around, it means the problem is caused by our shopping cart module.
Try it and run the same test again with the same conditions (without logging in) for validation, then let me know.
Regards.