Actually, google does have a problem with chitika. However, chitika ads by default are predefined key word based. (You can change this if you are not using AdSense). Yahoo's ad categories are different, they serve ads that ARE contextually matched, but just in a pre-defined category. For example, say if a page was about cars, however their catagory was internet services, they would only get ads from the internet service catagory, with the keywords from the car page. And if there are none(which is likely, since its about cars), it shows vonage or something like that.