Brandt adds it all up.

Google's webpage would appear to be the definitive list of all the possible reasons Google can think of for discrepancies between AdWords and another analytics system. If Google staff are trying to educate you, their procedures dictate that you be referred to this document. You may also, in my experience, receive emails from them containing points copied from it. I have yet to see any examples of Google staff coming up with any possible reasons that are not contained in this web page in writing.

There is another reason that may cause a discrepancy between AdWords and your system: People may click the ad, and then fail to arrive at your site. They may change their mind and hit the back button before your page is loaded. The Google re-direct may be slow and people may give up waiting (I've personally experienced this many times). I am sure there are other reasons as well.

Richard Holden mentioned this as a cause of discrepancy in our interview, and I've heard it from other Google staff in conversation. One wonders why Google have not put this point in their web page? Could it be they have no desire to publicize the fact that even if someone never sees your site, you still have to pay for it?

All of the points Google does make are perfectly valid, and can lead to a discrepancy between your numbers and theirs. However, no one complains to Google if the numbers vary by a few percentage points. The issue is whether these reasons can account for discrepancies large enough to trigger a billing dispute. In my opinion, if your system is implemented correctly, the maximum possible variation should be no more than five percent. I have personally seen variations of 25 percent with some regularity and recently a consistent difference of 70 percent. Nothing in Google's documentation can possibly explain such variations on sites where the analytics systems have been implemented correctly.

Conclusions
The key factor controlling the process of AdWords billing disputes is Google's unquestioning belief that the AdWords click-tracking and billing system is 100 percent perfect. According to Google your system has to be wrong because AdWords click-tracking is the most perfect computer system ever created by the human race. In this sense it is the absolute pinnacle of technical evolution.

In writing this article I have tried very hard to think of any other technology that even vaguely approaches perfection, but I can't. When you consider the amount of data it processes, you realize Google AdWords' click-tracking must be more accurate than an atomic clock! It defies the laws of physics in its perfection. Of course, you just have to take Google's word for it because it's so wonderful they can't tell anyone else anything about how it works to prove their claims.

Google does have a point; even if your analytics system is correctly implemented, some variation in numbers will occur. But if that variation is very large, you can take it from Google-- your system is faulty. Because Google AdWords is absolutely perfect.

If, by now, you still stubbornly refuse to be re-educated as to Google's Absolute Perfection, Google will happily remind you that if you want to use AdWords, Terms & Conditions stipulate you have to accept their numbers anyway. So tough!

Brandt Dainow is an independent web analytics consultant. Read full bio.