"...Our system can automatically distinguish between clicks generated through normal use by users and clicks generated by click spammers and automated robots. We are then able to filter out fraudulent clicks. Clicks deemed by us to be click spam should not be included in your earnings."
There have been many threads and posts from those who have been dropped from the Adsense program due to fraudulent clicks. I've seen the question, "Why can't they just filter fraudulent clicks?" asked over and over. Well, the answer is that apparently they can.