There can be different reasons of this behavior. Different image formats carry different information along with them. For example, a PNG can carry gamma along with it. Different software follow different ways to handle this. Acrobat 6, being an older version (a very old version actually) works on older algorithm.There weren't many advanced image formats available at that time. Acrobat 9 should be able to handle images with more details.
I can provide you more details if I can look into the data of some sample images that you have.