In the most expensive versions, they measure each individual patch, in case printing imperfections make the colours slightly different. And of course because you can’t get the exact same inks when a product is 50 years old!
> How were the colors chosen?
Skin and foliage; primary colours; greyscale; and miscellaneous.
They also make a 140 colour patch version - the “ColorChecker Digital SG” - that provides more coverage; 14 different skin tones instead of 2 for example!
Fun fact, though: You have to replace them every couple of years (or sooner) because no pigment/dye is stable, especially if exposed to bright light.
Having a test shot with one of these in it saves you untold amounts of pain later.