Merc Treads not reducing stuns/slows/etc.

Just finished a game where we had a Thresh with Mobility Boots and an Ahri with Merc treads and when they got stunned or slowed at the same time the duration was exactly the same. I checked masteries as well, and Thresh doesn't have any points in anything with Tenacity. I saw it happen 3 times where they were either stunned or slowed at the exact same time (Anivia and Frost Queen's Claim while side by side) were the two pumping out the cc.