Basically, they assume you’re using a retina quality (aka HiDPI) display where each virtual or abstract pixel maps to four physical pixels in the display.
For example, you have a 27” 4K display, but set the resolution to 1920x1080 in System Preferences.
Mapping each virtual pixel to four physical pixels lets you render curves really crisply. However, on non-HiDPI where you don’t have pixel doubling, you have to use sub-pixel rendering to approximate curves.
One technique is playing with slightly shading adjacent pixels so that at a distance curves appear to be smoothed out. But for some technical reasons, Apple dropped support for sub-pixel rendering. It was too hard to do and the future is HiDPI anyway.
For example, you have a 27” 4K display, but set the resolution to 1920x1080 in System Preferences.
Mapping each virtual pixel to four physical pixels lets you render curves really crisply. However, on non-HiDPI where you don’t have pixel doubling, you have to use sub-pixel rendering to approximate curves.
One technique is playing with slightly shading adjacent pixels so that at a distance curves appear to be smoothed out. But for some technical reasons, Apple dropped support for sub-pixel rendering. It was too hard to do and the future is HiDPI anyway.