Wasn't the previous Kindle the best selling product on Amazon also? I remember being surprised by that, because you would expect them to sell a lot more of a low value item (a book, an mp3 track, a CD) than these.
My suspicion is that this is true in some literal sense, but a bit misleading.
As to the side discussion on the "best" products, often the most popular are simply the best. I often research what to buy using what sells best on Amazon - it's the "wisdom of crowds", I guess (which is quite an interesting book btw). It seems to me that buying something less popular to prove to yourself how independent you are is no better than buying what's popular because it's trendy.