tristana jumps during blitzcrank knockup and ult (GAMEPLAY)

I was playing aram with blitzcrank (iblitzcrank skin) against a tristana. everytime i knocked her up she could still jump away, once even during the silence of my ult and at the highest point of the knockup. this happened every single time i knocked her up and she had her jump. i am sure that my knockup hit her before the jump was even started.