Well, in general ad networks show their highest-paying campaigns, followed by their second-highest, and so on. In other words, the more you run an ad network,

Well, in general ad networks show their highest-paying campaigns, followed by their second-highest, and so on. In other words, the more you run an ad network, the worse it tends to perform. It might be the case that the worst-performing iAd beats the best-performing AdMob campaign, but this seems unlikely - and if it's not so, developers could've optimized yield and made more money by working with both parties. Generally, excluding any partner from your ad rotation reduces yield, which is why many online sites will work with many ad networks simultaneously.