Why Most ROAS Tracking Breaks Down Before It Even Starts

Picture a marketer pulling ROAS for the same week from three places: Meta Ads Manager, Google Ads, and Shopify. Three different numbers show up. Not slightly different, either. Meta says 4.2x. Google says 3.1x. Shopify's order data, once you back into it, suggests something closer to 2.6x. Nobody touched the campaigns. Nobody changed the math. The spend was the same, the sales were the same. So which number is real?