Yes you are right but every site user try to make his default and yahoo and google also do like this. Make his site default at installing.
So I dont think something wrong there.
I seem to get a problem too, if it is right about that!
I will tell the little story:
On my school im checking adsense daily. And i told people about my websites. And they telled them their friends.
And since we have a lot of free time at our school, they are often surfing my websites and possible click the websites... I last signed in and discoverd a channel that had 1 click and no earnings so i think that had to be a click from school... there might be more clicks but im not sure... shall i mail google? and is there a way to block ads for a certain ip?