Despite the so-called "Death of AdSense" (which happens to be a smart marketing ploy), there are still a few good success stories. At least, the marketers who carry the right beliefs within them know what they are doing to persevere and achieve desired results.
One of these correct beliefs is knowing how bid pricing works. Generally speaking:
1) If there are not enough ads to go around, that particular niche is too small to try.
2) If the "general economy" of the ads is rather low, avoid the niche too. That's why there are high-paying keywords and low-paying ones.
3) If one site performs better than a similar one in AdSense clickthrough rates, that site will be served better paying and better performing ads. That's how smart pricing works.
We're sure Google has many secretive and subtle metrics to disqualify junk sites and the corporation insists on surrounding itself with webmasters who are committed to providing quality work.
Going back to point 2), no matter how genuine sites are in providing valuable content, webmasters need to know something about the state of the competition related to a supposedly high-paying keyword.
There is a general belief that "certain keywords pay highly" (granted), like bankruptcy, cancer, lawyers etc., but without research to back them up, such a belief does not stand on a foundation.