Facebook question for Shopify app developers. Help please!

[Cartmate]

Hi Colleagues
My team and I created a Shopify app and started promoting it through Facebook. We ran into a problem right away. Facebook captures 5 times more installs than it actually does. If you have any experience in setting up Shopify app advertising through Facebook please tell me what I did wrong.
I did the following setup:
1) Specified my Facebook pixel number in my app listing.
2) Went into Facebook Event Manager and specified that I was setting up the pixel through a partner (Shopify)
3) I then marked up all the buttons in my Shopify app. I attached a purchase event to the Install App button
4) Launched an ad with the goal of Purchase

I get 2-3 app installs per day, but Facebook captures 10-15. Maybe I'm getting the target wrong?
Thanks in advance