Back in 2004 Google started a policy known as "Smart Pricing." Smart Pricing basically recognizes the fact that click abuse and other end-runs around good internet practices can cost advertisers big-time. With Smart Pricing, advertisers pay less for clicks that come in from sites that are likely spammy or click-abusive. It just makes sense -- if I'm an advertiser and some jerk in China spends all day sitting around clicking my ad just to make a few dollars for himself or a click-abuse client, then I'm going to stop advertising with Google. They's very smart at Google, and they are always working to counteract the best attempts of those who want to exploit the money-making possibilities on the internet.
If you have an Adsense account, and you start getting a zillion page view and a bunch of clicks that are all worth a few pennies each, Smart Pricing may have kicked in. BE CAREFUL, because that means people, or you yourself, are click-abusing your sites.
Here's part of Google's announcement about Smart Pricing:
So be careful, be smart, and play by the rules!