I guess it varies depending on your viewers watch time. Let's say you have a one hour video with 4000 views, and all of that viewers watch your video from 0 to 60 minutes. So now you have 4000 watch hours. But if you have one hour video with 4000 views, and the all of those viewers only watch from 0-30 minutes, now you have 2000 watch hours. But I could be wrong tho.
If your video length is 60 minute then your video get 4000 watch time. So there's going to be some all kinds of watchers in it within 0-60 minutes. Some must have seen the whole video, some have seen 5 minutes. So YouTube counts the total watching time minute.