Google can and will lower your RPC when you have low conversions. You can talk to many site owners who's user base are young teenagers (12-15) and many will tell you that they only get in the neighborhood of .02/click for ads.

I have no idea how google accomplishes this, but the cases of this happening are well documented. This is one reason I tend to not show adsense ads to my members other than as filler for spots where monthly and cpm ads generally go.