Found the omission that made this paragraph hard to grasp...

Quote Originally Posted by Stephen Benskin View Post
The large majority of exposure determining devices make a measurement proportional to this arithmetical average and in consequence it is this quantity which is used in exposure determination.
It may be noted here that it has been stated that the geometric average of the scene luminance is the better measure for the purpose of exposure determination

Quote Originally Posted by Stephen Benskin View Post
, but because this quantity is no more closely related to the ratio of the extremes of luminance than is the arithmetic average, and moreover is not easily measured, it is not used in exposure determination for ordinary photography.