Movement Speed misrepresentations

Alright so I know many, if not everyone who played League, has ran into this issue before: You're chasing down a low HP target champion knowing it would only take one of your abilities to finish them off, their teammates are dead and there is nobody to help them. You right click ahead of the enemy champion to try to catch up so that you can apply the death blow, HOWEVER they are somehow outrunning you. When you check their items and stats you see this: Tier 1 boots and 385 Movespeed, they do not have items that grant %MS, you check your own stats: Tier 2 boots and 400 Movespeed. The end result is that they somehow end up outrunning you even with all their abilities down, frustration ensues and your computer somehow grows wings while your window has a gaping hole in it. Can we please address these Movement Speed discrepancies that are all too common? There should NEVER be an instance where a champion with lower movespeed should outrun a champion with higher movespeed. Movespeed as of now does fully calculate your total MS taking into consideration runes/masteries/keystones/items/abilities, so I'm wondering just how the hell does this keep happening where people outrun you despite moving much slower? Before people say it's the pathing, lets assume both players take the exact same pathing in all scenario's.