If you look at the cone response curves you'll see that each of the three has a peak at a certain wavelength and then falls off. At the blue end of the spectrum the short wavelength cone most responsive to blue wavelengths falls off quickly. But the long wavelength cone most responsive to red wavelengths flattens out and remains responsive well into the blue end of the spectrum. What this means as that at the shortest visible wavelengths the the long cone is contributing a relatively larger contribution as the short cone falls off. The result is that the blue becomes biased toward red slightly and we see violet. You can created the same effect by mixing blue and red wavelengths and get the various shades of violet and purple. The only thing that really distinguishes violet is that you can create it with monochromatic light, which you can't for other hues of purple and magenta. This leads people to say things like 'violet is a real color, while purple is only in our heads,' but, as Telecaster points out, this is mostly nonsense.