it is a bug imo, or at the very least poorly designed, as i've just tested using the factory settings, and if we define a background color for say the 3d view, by using the user preferences window > themes > 3d view, then go into the properties panel > world > world > horizon color and use it's color picker to copy the color from the 3d view, or the user preferences path, the result is the same color visually but with different values for rgb and hsv, unless the defined color is pure black or white.