[GAMEPLAY] Mordekaiser W movement speed?

I was just playing in an ARAM game and I noticed that mordekaiser's "magnet" mechanic with his W didnt seem to be giving me the full value of movement speed. It's supposed to be flat 60 movement speed, but instead I was gaining 50-ish movespeed whenever I was moving towards the target of the ability. After going into a custom game to try things out, however, I think this may be an intentional mechanic, some sort of "diminishing returns on movement speed" policy? With no movement speed items or anything, Morde has base speed of 340. Moving toward the target of his W at this level gives +60, up to 400 speed, exactly like it should. However, with boots, Echo, etc. I was only increasing from about 423 movement to 475 movement, a gain of +52. What exactly is going on here? How is movement speed calculated?