Karthus Q is sometimes slower [BUG]

Here in my video you can see how sometimes the Karthus Q is slower than other times. My video isn't manipulated you can see on Froggen's gameplay at min 4:04 and 4:09 https://www.youtube.com/watch?v=yrLDYdMLv4U

I hope you correct this. As you can see, in my video the big clip starts the channel before the smaller one, and in spite of that, the bigger clip's Q explodes after the smaller one. What the hell is this?