Google uses random ads. Just because an add is in the top position, doesn't mean it is the highest paid listing. They also randomize (I think) related searches. For example, if you have a "games" site. You may see sites that have adwords for "free games", "free game downloads","game downloads" etc. Since these are all different search terms, the bids are different.
I have a free web hosting site that on the first day, only had 10 clickthrus, but the revenue was $25
Since then, that site gets 50 daily clickthrus, but yet the revenue sometimes is only $10.
It just depends on how lucky you are I guess
, Plus there are other factors involved, I'm sure.
Hope this helps.