not giving full details does not mean it does not matter. Whatever Bookscan captures is basically the same for every book, so it does not give you total number of copies sold, but it pretty accurately measures relative success. Which is what the charts show.
The one thing it does not account for is digital sales / direct sales, but unless they basically doubled or tripled between two releases, to account for the difference in sales, and otherwise stayed flat, the chart does show a decline in sales.
While digital is growing, it fid not make that one bug jump at one point in time