If I were to see a word which gets 100 clicks and 90 unique IP's I would assume that is a few users clicking on ad's more than once. But, if I saw 500 clicks with 90 unique IP's I'd call Google to investigate.
I'd like transparency in this regard. There should not be (as far as I can discern) and "revealing the algo" issues.
The bottom line is, if they enter and do not surf past your landing page, or stay for 1 second, or come from the same IP you are onto fraud.
There are many statistical programs that will do this for about a dollar a day ;)
Most tracking systems will also use a session, or a cookie. Some cookies do not get through ,yet a lot actually do. So that is a possibility.
I would recommend you look into hitslink. Look good and hard. It is truly a great system and light fraud tracking was added about a week ago.
KG