Another exception would be an e-book that has sold well, but in a very exclusive market. There are lots of e-publishers, big and small, all over the web. Normally, big name publishers have a greater probability of selling more books than smaller publishers due to several facts. Economy of scale. It's easier and cheaper to publish and promote lots of books than just one or two.
Big publishers can afford to combine books into larger promotions.
They have access to other divisions, parent companies, sibling companies and so on to help get books out to the market. Leverage with the buyers. Bigger publishers have more leverage with the book buyers than small publishers, hence better distribution. That being said, it's not certain that e-book publishers experience the same conditions. It's very possible that a small but motivated e-book publisher will get your book out there much better than one of the big guys.