I knew it was possible to manipulate the figures - there are numerous websites to guide you through the process, most of which seem to have been written by business gurus keen to promote a how-to-get-rich-quick pamphlet. (Basically, you get a lot of people to buy at the same time - that way it is possible to break into the top 100 in a particular genre.)
April explains the mechanics of it:
ASR (Amazon sales ranking) is recalculated hourly, and only includes titles that actually had sales activity during the hour in question, or were ranked within the top 100 in any category during that hour. This is how it’s possible for your book’s ASR to sometimes be higher than that of multi-million-selling books by authors along the lines of JK Rowling and Stephen King; if neither of them sold any copies in a given hour when you sold 2 copies of your book, your book’s rank will be higher than theirs’ for that hour...Read the rest of her interesting post.
The reality is that there is an whole industry devoted to helping companies (or individuals) to fake it, before you make it...I recently discovered that you can buy twitter followers by the thousand (they won't actually read your tweets though - it's hard to manufacture genuine reader interest) and for all I know blog followers and probably lots of other things I haven't thought of...ah well, better to know than not know and read with a large pinch of salt handy...