OK, I admit that your explanation is possible (and I appreciate your comments, I learned something from this experience), but I still think that impression spamming is more plausible of an explanation. I mean it never occured to me to do something like that to drop competitors' CTR, but now that I know about it, I think I could write a script to do it in about 30 minutes (and I am slow :) ). Not that I would ever do it.
What I'd like to know if Google has any sort of safeguards against something like this. If not, then the placement formula taking CTR in consideration is pretty much useless.