Why This Comparison Keeps Coming Up

Here's the scenario, almost word for word, every time: a Shopify brand launches, Shopify Analytics is already sitting there in the admin, and it looks fine for the first few months. Then the ad spend ramps up, someone installs GA4 to track campaigns, and suddenly the dashboards disagree. Shopify says 142 orders. GA4 says 118 conversions. Nobody installed anything wrong, exactly, but now there are two "sources of truth" and they don't match.