The only thing I can think is that different size ads pay more as well. I have only been with adsense a week now, but in the 2 days I made money, I had 5 clicks one day that made $0.18, 1 click the next that made $0.18 cents. I'm thinking maybe larger banner ads cost more to run, therefor they pay out more per click? I don't think google knows what they're doing though because for a total they show $0.37, and last time I checked 18+18=36.
