Braum Passive vs Kai'sa Visuals

I'm used to playing Braum, so when I see that her passive auto attack thing is 4 stacks and then the expunge kind of damage I am a bit confused. I guess it is in line with other champions such as who all only have 2 hit marks and then 3rd is proc, and makes sense because the 3 stacks now means altered abilities, but why is braums 4 stacks but activates the stun on that 4th hit?

Only idea I have is it's because it's for hard CC and not just damage or damage/slow.