In Mac OS, because that's how its scaling is designed. Windows does not have this problem, because it tells programs to render themselves at 1.33 or whatever and provides facilities to do it without pain in the ass. As the result scaling in Windows is painless with any displays at any resolutions. In fact, my main setup has two 1.25x DPI displays and an old 1x DPI display with zero blending or sizing artifacts excluding brief moments when you move a window from 1.25x display to 1x one until it lands.