The difference comes from the fact that Google doesn't have as much information about the ads and people they were served to as Facebook does.
All of our revenue reporting comes directly from Facebook since they have the best information about attribution for the ads. Google Analytics and Shopify both only see things from a session point of view, so if someone sees the ad and clicks on it, but doesn't buy right away, then comes back and buys later, they might not attribute the sale to the original user.
Facebook knows exactly who saw the ad and who purchased, so their data is much more accurate, which is why we only use Facebook's attribution for reporting revenue.
For example, if someone saw the ad on their phone and bought on their desktop, Facebook knows who they are so it can attribute the sale correctly, where Google has no way to link the two events so cannot count the sale.
On our Campaign Details page, we also breakdown click vs view revenue. Click revenue is when someone clicks on the ad and then makes a purchase within 28 days. View revenue is when someone sees the ad and makes a purchase within 7 days. Google will only ever be able to report on click revenue since they don't know who saw the ad, and because of the session/device issue, the revenue they report will always be less than the true value you get from Facebook's reporting.