I was watching a video on Youtube and noticed the resolution option of the video going up to 4k, so I decided to select it, and I noticed that the video appeared to be sharper than 1080p and 760p. I was under the impression that my screen, which is 1366x768p could play only up to 760p of videos at that quality, and higher resolution videos are simply scaled down.
How is the image appearing sharper?
There's no such resolution as 760p, it's 720p.
You unfortunately have one of those red-headed step-child screens put out about 15 or so years ago pretending to be "HD" but without any of the benefits.
Every 'TV' picture you try to watch on it is going to be horribly scaled… because they chose that size as the largest that could be made in a 16:9 aspect ratio whilst still using a 1 megapixel chipset & for no other reason - price.
It won't upscale 720p properly, nor downscale 1080p. Your probable gain at 4k is the choice of downscale algorithm that just about gets away with it, pure luck.
If you have the budget, at least get a proper 1920x1080 'True HD 1080p' display.
I found a good rant about it here - 1080i on 1366x768 resolution problems from way back when 1080 itself was still considered 'luxury';)
This is hard to answer, since we don't know how YouTube generated all the versions of this video.
Even if we assume that the original video that was uploaded to YouTube was 4K, we still don't know which algorithm was used to downscale it to 760p and how much computer power was YouTube prepared to allocate to this operation (if we take into account the enormous daily upload of videos).
The conclusion must be that the algorithm that is used on your computer by your video driver simply gives better results than the one used by YouTube. Even if the same algorithm was used, YouTube might have parametered it to run faster with worse results than yours.
Also, as YouTube didn't use exactly the same resolution as your screen, what you see at 760p is the original 4K video scaled to 760p by YouTube and then scaled again to your screen. Each conversion will lose some quality.
When you're watching 4K video, it's only scaled once to your screen, so with a lesser loss of quality.
