More specifically any addition of color values generates the problem, not just averaging. If you add two linear values and gamma-correct the result, you get a different answer than if you added the gamma-corrected values directly. Multiplication doesn't have the same problem. I.E. a^x+b^x != (a+b)^x, while a^xb^x == (ab)^x. P.S. it has nothing to do with Nyquist limits.