One or two bidders on a spending spree can raise your revenue. Also, raising your stats raises your revenue. There are threshholds for ad searches.
My methodology involves having minimum prices, so my ads are also showing off stuff for Zazzle (and will eventually expand to showing off other things I have for sale too!) when I don't feel I'm getting my money's worth.
I must say, it DID increase my revenue from the ads, if even a little...