Jax Hax

I am genuinely confused.

Earlier today I was playing ARAM. I played as Singed, and one of my opponents played as Jax. Since I was tanking, I had a lot of Armor (Frozen Heart & Thornmail), but throughout the match I noticed he was killing me easily. But when I checked his items, I saw that he had absolutely NO Armor Penetration, yet he shredded through my Health like it was nothing. I checked his abilities as well, just in case. No Armor Penetration there either. Any explanations??

The Match History page is linked on this discussion.