Allow me to explain.

If you immediately use one of them and the Bloodcurdle won't be applied, that means you will deal more damage (12%) than supposed to deal (5%).

Both moves are ideal in their own ways and work exactly the same, however Whirlwind Pierce can be used only when the move's stun is to apply it (Headbutt, Shadow Rush, and if properly timed after a Downslam).

For Curdle Pierce, the Downslam timing is the same basic principle that you need to land it, either way you can use it after Aerial Downslam. There's no other move that has enough timing to land Bloodcurdle and don't let the opponent evade it, so it's just hoping that it will land after any Downslam means.

Sorry that i explained it too late, though!

Thanks tho, it kind makes sense now, and i also found out while experimenting that if you do m1 uppercut and then spam whirlwind pierce, it will 100% hit without timing but it won't let you apply stun.