Political books are a different animal than other books. You'll find political donors buying a huge number of copies to propel the book to the top of the charts to make a political statement. (my ideas can't be crazy if 50K people bought my book on the first day!)

Politicians use the books for fundraising and can end up with a lot of surplus books. I suspect that the donors that purchased Clinton's book needed to free up some garage / warehouse space and dumped them when they were no long a useful political tool.