Some might have to do with the camera. Daniel had a write up on the 40D I think. ISO200 is better than ISO100,and ISO50 is the 100 with some extra math on it. Something like that anyway. Cameras have places in the ISO range that give better S/N than their lower counterparts. It might still be in here unless it got truncated by the move to the new server. It was a good read with a link to a graph.