One of the things that troubles aspiring authors is understanding just what amount of books need to be sold to be considered a successful writer. Obviously, we understand that the publishers expectations and the amount of money spent come into play. But there are such massive disparities, even on the bestseller list that it can be confusing.
For example, we all know that J.K. Rowling sold something like 8 million copies of whatever Harry Potter novel in 24 hours. Meanwhile, back in 1995, it was thought that John Grisham had the fastest selling hardcover ever when he moved 300,000 copies in a week. And then you compare that with smaller writers, but no less successful in their own way, like Tucker Max who said he sold 60,000 copies of his book in six months. All three were bestsellers, but all three had dramatically different sales totals.
So given the wide-range of sales figures, and the fact that the public almost never knows how many copies a book has sold, I always perk up whenever an article appears that gives the statistics.