It's based on vision. If you have vision of the target by the time you land, you fire the arrows. If you don't, then there's no target and you shoot nothing. This is why you didn't hit Karthus, he was still inside the bush when you landed. Should be noted, when I say "landed", I mean the game has processed that you landed, not the animation of you landing. If you watch the video you posted, you can see that you character stops moving (Game processes that you landed) about 1 or 2 frames before Karthus leaves the bushSame thing with the wolves, but this is more related to the way you gain vision in the game.
In league, you gain vision in intervals (I mean gained vision from your characters max distance btw, not vision gained from something already in your vision radius IE someone walking out of a bush), not as a build up. So, your vision will increase and decrease at a rate of lets say 5 units, not one unit. This is to make the game a lot smoother and cleaner. However, when your vision is suddenly gained based off of you base vision (IE wall hopping), the game first processes your vision as soon as you have cleared the wall (the literal exact point you have crossed majority of the wall, even if your still a part-ways going through the wall) then slowly give you more vision based on where you land as a gradient.
To put it more simply, you have to be pixel perfect when hopping over the wall at just the right spot of the wall to have the proper vision to hit the wolves. This is why you never hit the far right wolf, even though sometimes you are in range after the hop. Also, you need to be right against the wall before the hop. The range of the hop is about 20 units after the wall, putting you in range. If you go over the video though, you can see at one part where you fail to hop the wall, but don't hit any of the wolves. That's because they are outside of you range if your touching the wall on that side.
