Why my purchase number in Facebook adsmanager is different from real purchase in Shopify

[Sammy3]

Hello, 

I'm a newbie and looking for support. My ads show different number of purchases and purchased items, while my shopify backend shows another number/item

I double checked my pixel and see that it is installed correctly. Don't know where the problem is, please advise

[Josh_Uebergang]

What difference are we talking about Sammy? How many sales on each over how long? Minor differences are possible over a period of time if various systems have different time-zone settings.