Basically, advertisers compete to have their ads shown on your site. Some topics have more competition than other topics. If you write a hub on a highly competitive topic the CPM will be higher than if you wrote on a topic that is less competitive. For example if you wrote a hub on "How to Choose the Best Car Insurance" you will likely boost your CPM. However if you wrote a poem hub about wind, you would likely have a very low CPM. If you write about a competitive topic you will start to see a higher CPM.
To find keywords with a high CPM you can use the Google Keyword tool. Be aware that keywords with very high CPMs are often times difficult to rank in. The trick is to find a topic that is searched often, has very few websites on the topic and has a high CPM. Its a balancing act, but finding that perfect keyword is like striking gold.