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This could possibly be down to the conversion window you have on your Facebook campaign (e.g. if views are included).
The visitor may have viewed the ad on FB (but not clicked to go to your site) at some point before purchasing which means FB can claim it as their conversion.
It's beneficial in a way to know that the FB post may have contributed to their decision but does mean extra work to identify that.
If you'd rather just go by the clicks, you can amend the conversion window to exclude view-related conversions.
Hope this helps!
This is an accepted solution.
Anders from the Shopify Social Care team here.
Jamie's point here is exactly right. If a person sees a Facebook ad for your product and doesn't click it (impressions), but later browses your website and decides to purchase the item, the Pixel is able to attribute this conversion to the Facebook ad the person saw. Third-party platforms are unable to capture these events.
To elaborate a bit more on the discrepancy, many third-party tracking providers use referrer URLs to credit conversions back to ads. Some providers may under-report Facebook conversions by as much as 40%. This is often due to the fact that roughly 40% of people browse Facebook using HTTPS instead of HTTP and when someone clicks on an ad on Facebook and converts on a site, the referrer cannot be recorded since they left an HTTPS environment and entered an HTTP environment. In addition, if someone opens a new browser tab and purchases there (ex: they click the ad, run to a meeting, then go back to the website after work to purchase the product), the referrer URLs will no longer be available. The analytics tool will consider the purchaser a different person and not attribute the sale to Facebook. Facebook also has a doc which explains the discrepancy here.
That all aside, adjusting your conversion window from the default 28 days would be a good way to adjust for this discrepancy!