That is my understanding too. I wish I understood more about the designer INTENT of the engineering values they use in their design models.
No matter, I really believe that Bill Burk's bottom line (post #57) is correct. The design intent was for a photographer to be able to meter, use the provided exposure recommendation, and have a reasonably successful exposure MOST OF THE TIME. Knowing when that will and won't work is the difference between a good photographer and a photographer who is risking failure.
That about sums it all up. I would add that that's precisely why we do testing with our gear and our film. It gives us the knowledge to get repeatable results without the randomness of these factors.