DrByte is right, both companies are tracking sales. And actually both of them are probably honest and report your sale correctly. The reason is that customer can click on your product from both comparison engines. It happens all the time, type in google product name and you see the same products listed in google from different comparison engines. You may click on one of them, then another. This way, not only two, but more comparison engines can register click to your site and as a result a sale (if customer finally buy something).
Maybe you know, majority of visitors comes to comparison engines from google and some of customers do research using more then one comparison engine. Therefore you may expect such problem from time to time. But if ALL your sales reported by both engines, then it’s error in sales code installation. How many sales you’ve got from comparison engines in total?