It can be many reasons... as much as people here post about $50+ clicks and shit - the reality is that Adsense uses smart pricing and will reduce the value of clicks on a performance basis - for example if your site is shit, if you content is shit (MFA), if your traffic origin is shit, and if your traffic is considered junk by Google - being that it is little value to the publisher... then they will penalize your click value...
I find CPC to vary too much. Since the ads are not necessarily about the site content but about the visitor history, there is hardly a way to know what is happening. I have 1cent clicks and 1$ or even more clicks... Go figure.