Andrew,
Good question. It's one that confuses many people. The answer is that there are two systems..one has the primaries RGB, the other RYB. The first is applied to the colors that are generated by light, such as rainbow, the second is applied to colors that are generated by pigments or other tangible color elements, such as the colors in a painting.

See http://www.colormatters.com/rgb.html for a full explanation