One of the first things newly published authors do is go "gaga" (aka "crazy, nuts, obsessive") over their Amazon rankings. The rankings change hourly, which makes it even more tantalizing to just go take a quick peek. And for the truly ranking-crazed, Amazon has sites for the U.S., UK, Canada, Germany, France, Japan, and China -- giving an already procrastinating author limitless opportunities to waste their precious writing time checking their ranks in seven countries. This can quickly deteroriate into a ratings-checking addiction. I've heard checking Amazon rankings called "crack for authors." It's true. I was an addict. I'm weaning myself off of them, but there needs to be a 12-step program.
What it boils down to is that Amazon rankings are like an auto accident -- you know you should keep moving, and don't slow down to look, but you do it anyway. Basically, Amazon rankings don't mean diddly-squat to your sales, it's merely a ranking of how your book sells versus other books, not how many books you've sold -- which is the number that your publisher cares about.
One of the best explanations of what Amazon's rankings mean (and more importantly, what they don't mean) comes from the venerable (and much missed) Miss Snark. As always, she's priceless. ; )