Are all the invalid clicks really being caused by bots?
Maybe people are just clicking on the ads.
Ads are being designed with tricks to entice people to click and therefore the CTR should be very high from time to time.
What is seems like to me at the moment is that I have been assigned a personal account manager who looks at my stats every 20-30 minutes or so and then arbitrarily removes just the right number of clicks in such a way (probably assisted by some algorithm) so that I get just about the same amount or revenue each day. Then every month or so it seems like the RPM is set a bit lower than the month below.
Am I being paranoid? I do love Adsense though :o)