It's how much you get paid for certain ads some adwords subscribers pay lots of money for ads and you get paid the more they have to pay for there ads.
I have a similar problem. 1 of my sites is Slovak language has similar earnings. This is GEO problem, because I am showing there only Slovak language ads, and there is not a big range of ads. My other sites are OK.
Now i got smart pricing again last week i got average 0.03 - 0.04 /click but now 0.01 that's very bad. Anybody happen?
If you want to read about what Smart Pricing is exactly check out http://www.problogger.net/archives/2005/04/19/adsense-smart-pricing-explained/