To those who are saying "oh you silly person advertising income is always seasonal": could you explain for the benefit of morons like me (1) why this effect wou

To those who are saying "oh you silly person advertising income is always seasonal": could you explain for the benefit of morons like me (1) why this effect would be so much stronger for iAds revenue than for AdMob revenue, and (2) why it would take effect so suddenly for iAds and not for AdMob? Thanks.