Slows can speed up enemy champions?

Alright, so my friend and I were messing around in a custom game as Sejuani and Zilean, trying to see if Stormraider's Surge was effective on her combined with her passive slow resistance. I bought Boots of Swiftness and leveled to 16 (when her passive maxes out). Thinking that slow resistance stacks additively, I procced the mastery on Zilean and was subsequently "slowed" by him, only to find out to my surprise that my movespeed actually INCREASED. Movespeed before slow was 395 (340 base plus 55 boots), with Surge but not slow, 507, and with Surge and slow it was 558. We tested this multiple times, the only variables to movespeed being Surge, the Zilean slow and the boots of swiftness, and got the same results. So... report Zilean for assisting enemy team?