Having more than 135% slow resist will increase movement speed from slows

Taric proc-ed Phase Rush. Had Boots of Swiftness and the rune Unflinching.

I know phase rush and boots of swiftness stack additively but i'm not sure about the rune. Still when it gives you the maximum amount of slow resist, this happens. Update: It doesn't only happen with 135% slow resist, but also with 125%. You basically only need to proc Unflinching by using a summoner spell, and if you have more than 100% slow resist, the 15% bonus from the rune will increase your movement speed.