The Amazon sales numbers are wildly unpredictable, besides being easily gamed by anyone who wants to do so. Get twenty friends together to order your book at the exact same time and watch your Amazon number shoot up the charts!
A book that sold 100 copies ten days ago and nothing since, and a book that sold nothing for the last ten days but 100 today, will have sales numbers that are orders of magnitude apart, even though each sold the exact same number of copies. Amazon doesn't report sales through any other channel, which is another degree of uncertainty. Add in the facts that Amazon adjusts its algorithm at random intervals, and that they apparently add, subtract, multiply, or divide by a fudge factor to keep B&N from knowing how many copies they've sold.... It all leads you to the conclusion that Amazon sales ranks are there as an advertising tool for Amazon, not as a source of information for anyone else.