Lee, I agree with what you said above. As I was assembling some lamps today I was thinking about it and came to the same conclusion. But, something else occurred to me. I think the beam basically illuminates the mirror with a two dimensional distribution that has a long tail in the direction of the cutting assembly and may look like a gaussian in the direction normal to that. As the corner mirror moves farther from the upper left, the illuminated area becomes bigger due to beam dispersion. So if the spot where the middle of the beam hits were to become less reflective, then as you approached the upper left hand corner and the beam became more focused on the "bad" spot, you would reflect less power. As you moved away from the corner, less of the beam's energy would be reflected by the "bad" spot and you would, paradoxically, get more of the beam reflected to the cutting assembly. That and a buck will buy you a cup of coffee.
ULS 60W, Zenbot
Corel, Imagaro, Paint Shop Pro, Vextractor, Mach 3, SheetCam