It's not really surprising that sunny 16 is accurate. The sun as a light source is a constant. The only thing modifying it is the amount of of diffusion added by clouds (and to a lesser extent, pollution in the air).
Originally Posted by BradS
The only variable is in our abilities to recognise it.
Whilst we might not be able to accurately measure actual light levels using our eyes due to the fact that our irises open and close to compensate, we can recognise contrast which is also dependant upon the diffusion added by clouds.