On the advertiser side of Google's services, I have a refund for "invalid activity" on almost every monthly statement going back nearly a decade.

> You received a credit because we found invalid activity on your ads. What is it: Invalid activity refers to clicks and impressions that we suspect aren't the result of genuine customer interest. We don't charge you for invalid activity on your ads. Example: Invalid activity includes clicks and impressions performed by automated tools, as well as accidental clicks – for instance, if someone double clicks your ad.

Sure, but you'll never be in a position to audit that data. No independent 3rd party verification would ever do for Google ads.

one possibility is the refund is done to create the illusion click fraud is being prevented . It's like a manager telling the owner only $100 was taken from the til but $150 was and he pockets the $50. Not that I'm saying google is like that but you have no way of auditing this figure.

I get your point but in your example the scam would be:

'It's like a manager telling the owner only $150 was taken from the til but $100 was and he pockets the $50. Not that I'm saying google is like that but you have no way of auditing this figure.'

But, in googles case it's more like: We charge you 1,000$ they say 100$ was fraud and refund you 100$ meanwhile 200$ was and they keep the 30% of 100$ distributing 70$ back for fraudulent clicks they don't really investigate.