Have a look at your Google Analytics and Shopify Analytics. It's possible sales came from other sources. Another factor to consider is whether you have an app that affects the checkout process, mostly the thank you page. Upsell apps often modify how the pixel fires upon a successful order.
That means that Facebook attributes 6 sales to its ads (view or click attribution). The other sales either came from totally other sources or people used another device to buy after seeing your ad on an initial device. Attribution is a tricky thing.