It happens only when your ads get clicked in another Country. If you are in the US and someone clicks on your ads in Poland, you will get always just 1 Cent for that click.
It's common man.IT completely depends on the clicks generated by the users. You can't expect every user to click the google ads.
Yea I saw a drastic decrease yesterday, which seems a bit odd because while their is fluctuation in earnings for everyone - a sharp decrease without any traffic differences is a bit odd. I'm not going to say it's Google conspiracy..but it just seems odd that they're stock took a nosedive 2 days ago - maybe they're being more frugal (sp?) about what ads they serve (keep the higher ones for Gmail, etc). The thing that startled me was that I went down by more then 50% of what I usually earn per a day, without a decrease in traffic/bandwidth etc. Either a huge amount of people just learned about adblock yesterday or there's something beyond our webmasters control in terms of adsense revenues (the latter is where I'd put my money on - literally).