Almost certainly they're based off a rolling sales period.
Consider this: Every single different SKU of an item gets a different code. That means the "top end" is more fragmented than you might think, in terms of electronics sales (and something like a new board revision for the 360 usually comes in a different retail package, for example). Also, Amazon is the *only* source of the Kindle*, whereas other items are sold by lots of places.
(*Okay, some shops, but the vast majority will be direct sales)
So the relatively high DX ranking isn't that surprising.
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