Quote Originally Posted by David Lyga View Post
Well, the amount of change in intensity might well be off.

Actually this discussion might be at least somewhat analogous to a situation whereby flash exposure depends upon the reflectance of surroundings. Take, for example, the following: you wish to photograph a grey scale that is exactly 10 feet from the camera. The environment is in complete darkness. With flash on manual, you judge the proper aperture and fire. Now, if you are in a small room that has white walls and white ceiling, you are going to get a different rendition of the grey scale than if you were outside, at night in an open field with nothing to reflect upon. The grey scale in that outdoor instance would be considerably underexposed.

The following attachment shows part of page 416 of my (desk edition) FOCAL Encyclopedia of Photography under the category of 'exposure'. - David Lyga
Aha, "Lightening the shadows". That's the main reason.