OK. Another observation from Vex on his 3rd week with adsense: Those who know my previous messages would remember: I used to get like 60-cent clicks on average. Yesterday and the day before yesterday I got like around 8 clicks and yet just a few cents. I thought, "Oh sh**; I have been smart-priced, too." However, minutes ago, I logged on to my account again, and saw there one #.##$ click. This time, I said the same thing: "Wtf is going on?" The thing is just when I thought I was smart-priced and got the lowest paying clicks, I got my (probably) highest paying click. So I was probably not smart-priced. But then why were those cliks so low paying? Was that because people from Asia clicked on them rather than North America. Coz when I got those clicks, it was sleeping time for all North America.
It could just be that different ads won the "auction" for a spot on your site, and at one point you had cheap ones and another point more pricey ads.