Is that CPM in the HubPages ad program or in AdSense...?
For AdSense, the greatest thing that impacts CPM is the topic of your article. Advertisers choose keywords that they want their ads associated with, and they bid a certain amount (the cost-per-click) to have their ad appear. More competitive keywords can cost dollars per click, while less competitive keywords can cost pennies per click. If your content involves a high competition topic, then your CPM would therefore be higher because you'd get more earnings per click.
For the HubPages ad program, it's less straightforward. However, advertising rates are often related to a number of things: topic, traffic source, and geography. As with AdSense, presumably certain topics would have a higher CPM because they're in higher demand by advertisers. Traffic source may (or may not) affect CPM, because traffic quality varies between search engine, social media, or referral traffic. Finally, advertisers typically pay more for visits from readers in the United States and/or Europe, whereas they'll pay less for visits from other parts of the world.
Mind you, those are just some general statements about online advertising - I'm unsure whether they specifically influence the CPM in the HubPages ad program. But at the end of the day, the CPM you earn is based on the rates HubPages earns for the ads, which most likely varies across one or all of those factors.