Because her movement is more graduated then Koki's If you try to run left with Sami, you wont reach your top speed until about 1.3 seconds. but Koki meets his top speed at about 0.13 seconds, and is more free in the air then Sami. Sami also takes longer to stop running and turn around.
To put it short and simple, Koki's movements happen more instantly then Sami's.
To put it short and simple, Koki's movements happen more instantly then Sami's.