if it's still relevant, here are some ideas what the problem can be.
1. Check your Pixel and the Purchase event - does it fire only once when the purchase has been made? It might happen that the event is connected to more actions (when page is loaded, button etc) and therefore the event might be twice fired.
2. Attribution model - Facebook's default attribution model is 1 day after view and 28 days after click. This means that Facebook is attributing purchases after quite a long time - e.g. A user clicks on an Ad on Facebook and is redirected to your side. Then surf your products and decides to leave your website. After, let's say 10 days, the user visits your website (organic - directly writing your website) and makes a purchase. Facebook will attribute this purchase to the ad which had been showed to the users 10 days ago. So, yes, the numbers might be very misleading and the real numbers are those which matter.
Hope it helps somehow to tackle your issue.