[jfengel]

Because the cone isn't really a "blue" cone, and neither is the "red" one. The curves overlap in complex ways. A pure violet photon also slightly stimulates the long wavelength cone.

That's why red+blue=purple feels a bit like violet. It creates a similar double firing.

(And why red plus green gives an even more accurate yellow. The long and medium cones have a lot of overlap.)

[GrantMoyer]

This is a common misconception, but the sensitivity of L cones ("red" cones) increases monotonically until about 570nm (monochromatic yellow), so violet light stimulates L cones the least out of all visible wavelengths of light. Magenta light, a mixture of red and blue wavelengths, stimulates L cones far more than violet light. See Wikipedia's LMS responsivity plot[1] or the cone fundamental tables from the Color & Vision Research Laboratory at [2].

I think the misconception comes from plots of XYZ color matching functions[3]. The X color matching function indeed has a local maximum in the short wavelengths, but X doesn't represent L cone stimulation; it's a mathematically derived curve used to define the XYZ color space, which is a linear transform of LMS color space selected for useful mathematical properties.

[1]: https://en.wikipedia.org/wiki/LMS_color_space#/media/File:Co...

[2]: http://www.cvrl.org/

[3]: https://en.wikipedia.org/wiki/CIE_1931_color_space#/media/Fi...