they are probably talking about their caching. Videos with low views are not cached at all servers, especially not higher definitions. So you want to watch the video, the server realizes "oh hang on, I threw this one out, let me grab it again from another server". while the server fetches the fullHD video, you get it in 360p

I recently had a video with very low views that just wouldn't load for minutes on end. Was wondering if they were down, or if it had to be grabbed from some tape archive because a disk crashed. After about 10-15 minutes, it slowly loaded. This was at all qualities, browsers, and even when trying from another continent.