It doesn’t actually take much to get to the top of that list. The chat is updated hourly, so it just means lots of people bought the book in the last hour (when it cost pennies like previous poster said). Amazon charts are meaningless when it comes to popularity (according to my friend who works in publishing anyway). You can have a book that doesn’t sell very well overall get to the top of the chart.
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Just checked again and it’s already back down to number 7