Color Wheel Question

Blog reader Emily emailed me a question that I couldn’t answer, so she gave me permission to post it to the group mind.

"I was opening up a new file in Painter today and started thinking about that little color wheel it shows you - the standard computer color wheel with all the colors in the wheel and the brightness and darkness controlled by a slider on the side. I noticed that the blue and red seem to have the illusion of spokes of more intense color poking into the wheel and the teal, purple, and yellow appear to have light spokes poking out of the wheel. Only the green just looks nearly flat all the way across. If you desaturate the whole wheel, all the tones are perfectly even. Any idea why this is?"