The big problem with third-party auditors is that they continue to count “fictitious clicks,” clicks that Google doesn’t count as clicks at all, in click fraud estimates.
Here’s one major example: Users click on a Google ad on Google.com or an Adsense site. When they land on the advertiser’s site, they click on products, hitting the “back” button to go back to the landing page.
Many browsers reload the landing page each time. We don’t count those as clicks, but third-party auditors actually register each click on the “back” button as another click on an ad, which grossly overestimates the number of ad clicks. Link.