Cut setting = bad quality

Hello everybody,

I think VDownloader is very cool. But i've got a problem here. I would like to download a video from Youtube, but not the entire video, just de first half.

So i use the Cut setting : 0 -> Half

But if i use this option, the quality of the video is pretty bad. If i don't use the cut, it's perfect. I use the "Best quality" to download my file.

Please, can you tell me why the Cut setting low the quality of the final video ? Thanks.