Not sure, would need more information to figure out why:
- what version of virtuemart are you using
- moving from the normal shopping cart page all the way through confirmation page, does the discount stay the same?
- what is your tax rate?
- what tax mode are you using?
- are you displaying prices with tax?
- can you provide the numbers of the example above where you receive 16.31%? subtotal, total, shipping, tax,.... every number on the screen