Display RGB attempts to match the RGB cone response of the human eye. Much of what goes on to create "colors" on displays is, to some degree, trickery to fool the eye/brain into thinking that its getting a normal response, when its not.
Thus, "pure" yellow stimulates both R and G cones, by matching the same proportionality of response with R and G emissions from a display, the eye/brain gets similar stimulus and thus "sees" yellow.
TTFN