Karthus Crit Bug

Pretty much is exactly what it sounds like. However, the Karthus wasn't ad. Full ap Karthus was critting for ~1281. In the screenshots attached of my advanced match history you can see that he had no crit items, was full ap, yet was critting. I messaged Riot prior to coming here, and they did a mini investigation and came to the conclusion that it's simply a bug. Anyone else noticed something similar?

... no crits for you.