Don, the base for 35mm is 5mil, for 120 is 4mil, but for sheet is 7mil. I think this means that it has to have some kind of color added to the thinner 35mm base to get the tint.
I was thinking about the blue(35mm) and brown(sheet) color that phil mentions vs. the neutral grey I get on all formats. I use D76 and he was using Pyrocat which I understand leaves a usually green stain. I have never used Pyrocat HD but could this be the difference?