User blog comment:Care Level/Once again, on Attacks v. Spells./@comment-1330314-20150808203947/@comment-9593635-20150808212733

The difference comes down to kits and itemization. Regardless of how much AP Karthus builds, his ideal course of action is still almost never going to be "I should just AA them", unlike, say, Caitlyn; his AP-scaling AA is almost 100% for last-hitting, and the primary difference is in making ranged AA harass easier to itemize against.