Simplest reason it that 18% reflectance just is not the middle value of a glossy silver print.
Originally Posted by markbarendt
More detailed explaination is that each type of paper has a different maximum black (remember all the threads on Dmax ). The D-max, then determines what the D-middle (D-Middle = 1/2 D-Max) is going to be. It is not one universal value and is highly dependent on the paper.
The "Zone" followers are always posting transmission log D values of their negatiaves and assigning them zones, but they almost never divide up the paper reflection densities into the appropriate zones. I don't know why they leave this step out. But if they did they would find that the middle is about 36% for paper with a D-max of 2.0.
The 18% card would match the middle value of a paper with a D-max of 1.48.
Again, the 18% card is an exposure tool for times when you need an approximate incident reading and have only a reflected meter.