@green:
this is normal behaviour because of discounts being added on the checkout page. i suggest you
1. turn off the "shopping cart"-block for "cart/checkout" and "cart/checkout/*" in drupal's block configuration
2. go to "store administration->configuration->checkout settings->edit->checkout panes" and disable "cart contents".
3. if you translate your shop, you can just overwrite "Subtotal" with "Subtotal (net)" in your language...
a little hacky, i know, but it satisfies my needs 
have a good time
...sebi