There's a difference between projected light and reflected light.

Web, like you're looking at now, is projected. Here's a thought - there's no way to project black. All the black you see on your screen is an illusion. It's just the absence of light.

Print is reflected, thus CMYK. Basically, when light is projected on any surface, some rays are absorbed and others reflected. The reflected ones are the colors that you see. Black pigment absorbs all projected light. White pigment reflects all projected light. Yellow pigment reflects yellow and absorbs all the others.

As a publisher for both web and print, our graphic crew is hypersensitive to the difference.