Sarah openly buying copies of her book from the publisher is a lot different than making bulk purchases from a retailer. Purchases directly from the publisher don’t count in the numbers on the best sellers lists.
Besides, over 2.5 million copies of Sarah’s books have been published and I doubt that Romney’s will do 1/10th of that.
Where can you find the list of number of books sold? I’ve seen it reported, but never been able to find the numbers online.