They're unusually good because 1. encoding research wasn't done on a business schedule 2. isn't done according to objective metrics (the main ones used, SSIM/PSNR, suck and you have to use human raters) 3. more varied and weirder testcases (some pirated movies, some video game screen recording, more anime).

There's a lot of other free video encoding tools, like avisynth plugins, that are just better than all professional tools. I'm not sure why this is, maybe customers aren't sophisticated enough.