for some time YouTube has been trying to improve the resolution of the videos that you see on the video sharing website. most of the high-resolution are not yet directly available but if such versions exist you can view them by upending &fmt=6 at the end of the YouTube video link. For instance look at this without upending the sting mentioned above and then look at this after upending the sting. In this video, especially in the smaller window, you won’t be able to notice much difference but if you enlarge it you will see it. Not all the videos are converted yet; after all there are millions and millions of videos at YouTube. Source.