How does Neeko Passive really work?

Ok you see, before I use my passive as let's say I have 320 movement speed. But then, if I disguise myself as my jungler, I have around 400 movement speed. I thought it was because I was in disguise, but if I changed to my adc, I had actually less movement speed than my own (Neeko). So I check my teammates movement speed and it's not the same as the ones I'm disguising as...

I hope this doesn't sound confusing, but I can't find anything about this anywhere... pls help [sg-janna]