My theory is that the CPA was high because iAd is a new and captivating ad-platform. Users who see AdMob (or other older-platform) ads are less likely to click

My theory is that the CPA was high because iAd is a new and captivating ad-platform. Users who see AdMob (or other older-platform) ads are less likely to click on ads. However, most users haven't seen iAds and so they may click on the ad just to explore it further - this means that a lot of users may click on iAds even though they may have no interest in the product being advertised.