Why couple changes to AP Items with the ADC change?

With the new items buffing adc's dueling potential and their ability to survive burst (New PD isolation mechanic and deaths' dance that makes 20% dmg turn into a 5s dot). I know that adc's are very item dependent which is why there are changes to items are needed to increase diversity. But why the AP item changes that seem to reduce damage output. Or was the intended change to make AP champions spike harder in the midgame and fall off more in the late?