Assume midday, with unrestricted sunlight. In neither case is the actual sun obscured by clouds. Now take these two scenarios:
1) Scene fully lit under completely blue sky
2) Scene fully lit under sky with many white clouds (but, again, sun, itself, is not obscured by clouds)
Do the scenes require the SAME exposure or does the scene with the completely blue sky demand one stop more exposure? I think that the 'blue sky' scene does require more exposure because there is no benefit of the white clouds 'filling' in with more light. And, there are (British) photo books that attest to my assumption.