On the Amazon Associates forum, a well informed lady said that AdSense staff who participate in a bonus scheme [I didn't know that] are absolutely livid with Google search for the monumental screw-ups.
Well for me the new rate would've helped me straight into bankruptcy. From €3,- Tuesday to €18,- yesterday, now in the twenties so far today. Slowly going back to "normal" as somehow with my site loosing all it's serp positions Adsense seems to have lost targeting as well.
I would expect this to result from a small sample size. It's the number of clicks which you're receiving that matters. Unless you're receiving 100's or 1000's of clicks per day, the effect of an occasional click worth $20 can show up as wild fluctuations if the vast majority of your clicks are worth just 10c.