Ad platforms grade themselves on a curve. Meta says it drove the sale. Google says it drove the sale. Amazon Attribution says, actually, it drove the sale. Add up what each platform claims and you'll often find your "combined" ROAS is 20 to 40% higher than what actually happened, because three dashboards are taking credit for one order. That's the core problem ecommerce analytics with omnichannel ROAS tracking is supposed to solve, and it's the reason most spreadsheet-based reporting quietly lies to founders every month.