Earlier quoted context omitted.
Most online businesses have multiple ways to track conversions... It will become pretty obvious if Google says you sold 130 widgets but your warehouse guys only delivered 60.... Likewise, lots of businesses have a different landing page for people coming from an ad vs people from organic search and people coming from referral links/shared URL's. The business counts how many people visited each of those. They'll know…
The issue is not that you sold 60 widgets but Google tells you it was 130. The issue is that you sold 60 widgets with ads but you would have sold 59 widgets without ads: are you making sales because of ads or are you showing ads to your would-be customers.
For brand lift campaigns though, without enough historical sales numbers, I can see how it’s harder to tell if your ad money is being well spent.