They kind of lie to us a bit in school when they teach us that to simplify the actual case. Other people have posted the actual curves.

L cones are most responsive to yellow-green, not red.

M cones peak in the green-cyan range.

S cones are most sensitive to violet rather than blue.

The whole RGB and CMYK mixing works observationally for us. But the biology behind it isn't as neat and as clean as the story they tell us when they're teaching us about the primary colors.