If you mix 50/50 red & blue, obviously that's purple.

When it's 80/20 red & blue, that looks more red than blue but looks purple but not the same as above.

When it's 20/80 red & blue, that looks more blue than red but looks purple but not the same as above.

It's a classification using a gradient, not an integer so yes, the boundaries are hard to tell but are still there.