It is the difference between additive mixing and subtractive mixing. When you mix colors on a screen with RGB, you add light. When you mix pigments on a physical medium, you subtract the amount of light reflected (because each paint absorbs most light except the colors it reflects, which are what you see).

As a side note, when mixing in the subtractive color system, your primary colors are cyan, magenta, and yellow. That’s why a printer takes CMYK, for cyan, magenta, yellow, and black. In case you were wondering, ‘K’ here is black.