Click Fraud of nearly a billion dollars....

Click Fraud is hard to fight against, which means it's hard to detect. Therefore I fail to see how they come up with statistics for something they can't even measure. Bogus in my opinion.

Actually, while you cannot come up with a perfect percentile--- and I think the study didn't go into enough depth (see: different industries will obviously have a huge variance in click fraud.) you can look for patterns in behavior and then take outliers and make an assumption that the outliers are fraud.
Think about it this way: When you have a credit card and you suddenly start spending way outside of your normal spending patterns-- an action is generated on the CC end and they give you a call to verify that you still have your credit card and are aware of spending.