I don't understand how this happened (consistently) with slows and MS

Consistently in my last game, playing as Quinn, the (fed) enemy ezreal would manage to use his blink followed by an IBG slow to get on me, and stick to me while chasing me.

What I don't understand, is I should have been FASTER than him despite the slow, because I used my Youmuu's active in response...

Quinn base MS: 335 Ezreal base MS: 325

Quinn MS items: Boots of Swiftness, Statikk Shiv Ezreal MS items: Boots of Lucidity

Quinn MS modifiers: IBG slow -30%, boots of swiftness reduces this slow by 25%, Youmuu's active +20% Ezreal MS modifiers: none

Quinn final MS: 335+55+335x0.05 -335x0.3x0.75 + 335x0.2 = 398.375 Ezreal final MS: 325 + 45 = 370 Also note that no Cloud Drakes spawned the entire game.

I should have had almost 30 extra MS, and yet he consistently was able to catch up to me by just running (recall, he already used his blink to snag me with the IBG proc). Is it possible that the slow is applying to total MS but the speed boost only applies to base MS? Or what else could be going on here?

He was never in range to use his BOTRK active. I don't believe he used his Quicksilver active to chase (he definitely didn't use it every time). Muramana and Dominiks don't give MS...