I would too. I was just looking at how the Sunday Times compiles its bestseller list:
"The way it works is this, each week booksellers across the UK and Ireland submit their sales figures to a market research company called Nielsen’s Bookscan. This must happen each week within a very small window, something like between 10pm Sunday and 10am Monday. It must also be submitted in a very specific way, a csv file that is analysed by a computer and the information fed in to create the results. Late entries are not accepted, and so authors and publishers are really at the mercy of booksellers to submit that data to be in with a chance of making the bestsellers’ list."
So it seems that the data is collected from booksellers (including Amazon) but not from publishers. A lot of us (thousands, I think) bought Graham's book directly from the publisher so that might not be counted in total sales.