Just by looking at the order for 2009, it looks like they take the order of the most successful consoles, and flip it over. I'm guessing they dock the most successful systems for having the most shovelware because they have the most games? Seems like a weird way to order them, since the bottom 50% of any system's library don't affect people's system purchase decisions in any way.
Or if I'm wrong, how do they decide these power rankings? I haven't heard of them before this thread.