Just an observation I made recently that left me a bit perplexed, so Im wondering if anybody can shed some light on the matter for me. My adsense account has about 30 sites linked to it and I generally get a click through rate of 8-12% across all sites. I know the approximate price per click each site receives, but every now and again I get smart priced without making any changes whatsoever, and that includes on both the sites and in the adsense account. How then can site A, for example, be earning £2.00 per click, and other sites B,C,D, etc earning their amounts, then all of a sudden site A starts earning 10 cents a click and all the other sites fall retrospectively? I would understand if I made any changes, but I don't. I also understand that a lot depends on user interaction once the click on an ad, but surely that wouldn't affect all thirty sites?
Maybe the eCPM shot down for that day? Try reading this - http://adsense.blogspot.com/2005/10/facts-about-smart-pricing.html
Smart pricing affects your whole adsense account meaning that if one site is smart priced, then all other sites which use the same adsense account will be smart priced as well. That's why you need to remove adsense from low performing sites to ensure you're getting the max cpc.