system
A chat on subreddit /r/pcmasterrace
human
Exactly. If your screen resolution is not evenly divisible by the video/whatever resolution, the pixels must be interpolated to fit on the whole screen. 

But even if it divides evenly, Windows or your monitor still makes them blurry because of the upscaling methods.

 The method where one pixel is just doubled, tripled, etc is called "next neighbour", so if you see this you can give it a try. Doubling only works if you use a 4k monitor and a 1080p video output or a 1440p monitor and a 720p video output tho, as these are the typical video/game/monitor resolutions (but you could also use a 1080p monitor and a 540p resolution, but that would look atrociously pixelated and most games nowadays don't support perculiar small resolutions
gpt
When streaming was pretty up and coming (when Twitch was still called Justin and Own3d.tv looked like it would be THE platform), I usually streamed at 540p to get 60fps output. Fantastic compensation since the video looked all right on everyone's 1080p screens.