There's a nice example in Photographic Materials and Processes that might help to clear things up. "If a hand held meter with a 50 degree angle of view is used to measure the reflected light from a given surface, the luminance obtained will be identical to that which will be given by a spot meter reading on the same area at the camera position. The reason for this independence is the fact that as the amount of light being measured from the original area decreases with increasing distance, the projected surface area included in the angle of view increases in direct proportion."