Well, this used to be the case, I know I've done my share of calculations.
It looks like things may have changed recently. I just found this article:
http://www.thedrum.com/news/2013/08/02/amazon-cuts-publishers-affiliate-commissions-earned-automatic-link-tagging
I don't believe we were informed about this change by VigLink, but I will look into it. Thanks for bringing it to our attention, Jo!