I am assuming because the top of the hand showed more dramatic changes. Afterall, most people mainly rub their hands together to wash them so the palms would probably be the first part to get cleaner with any friction. Just conjecturing.
While your conjecture might be correct, it shows how misleading it is. Instead of showing the part that matters, they show the part with the bigger difference, which has far less relevance to germ-spreading. I have a degree in research, and this (i.e., twisting things to make them appear relevant) is a huge beef I have with the research community.