"I'm not even sure it's a good indicator of what's selling on amazon"
Well, it's a nice indication of trends. If a console is a top-selling item for a longer time ( while competitive consoles are not out of stock of course) , you can expect to see US sales-numbers following the same trend.
For example, the difference between xbox360 and ps3 sales is a lot smaller now, you could see something like that coming a week ago at amazon.