Not sure what you mean by photoshops densitometer. Im assuming you mean the little readout on CMYK/RGB values. What colour model are your images in, CMYK, LAB, RGB etc? I think it is quite reasonable to have a small shift if your images are in RGB. But CMYK and LAB should be fairly stable (at least you would think that). Do you have any profile embedded in the file? And what gamut do you have your monitor at. Default on PC is 2.2 and it is 1.8 on the mac. Maybe this is the cause of the shift.

Beth