Also, from what I have read most of the versions of Windows 7 bought out there were with new machines that were replacing underpowered XP PCs, so the change between what the owners previously had and what they now have would play a large part.

Finally it should be taken into account that these types of studies if done on a small enough group of people could have a margin of error upto 4%. If the case is that for this study, that means that people are equally satisfied with both. If that is the case I think this is pretty much a non-story.