RGB screens can't display the full range of colors that our eyes can see. To produce a color that looks the same, you have to mix red and blue, but in real life, violet does not contain any red light. It's the same for any color that isn't pure red, green, or blue. For example, yellow is displayed using red and green, even though there is a single wavelength of light that looks the same to us.
P.S. Purple and Violet are not the same color. This is propaganda which is spread by crayon manufacturers.
According to Wikipedia, there is a wavelength of light that can activate both red and blue cone cells, which means it's technically violet light, but our eyes don't detect it very well so it appears pretty dark. Usually if you see something violet, it's a mixture of red and blue