User blog comment:ClariS/AD has Diminishing Returns!!/@comment-4636093-20120829022740

It's not direct diminishing returns like on Movement speed, where each point of MS is physically worth less when you get a lot of it.

It just means that each point of AD is less signifigant when you get a lot of it.

It's why APs tend to fall off unless they have ultra low cooldown spam like Karthus or Cassiopeia. It doesn't happen to ADs because they have multipliers to their damage Attack speed and Crit chance.