Say you have a UI element that's supposed to be 9px wide. If you have perfect 2:1 pixel ratio you can use 18 physical pixels and it's as sharp as the monitor can be. If you're using UI scaling of say 1.5, the monitor needs to use 13.5 physical pixels to render it. Except that's not possible, so it will average together the 13th pixel with the 15th pixel (simplifying). It's basically introducing an extra anti-aliasing pass to your frame. This is is why the Apple/LG 24" is 4k and the 27" is 5k. All that said, I'm sure 4k @ 27" even with 1.5x scaling still looks great and you'd never notice the difference, except if you had the two right next to each other.