indeed, more k-books are now sold than .epub books precisely because people know the kindle-books _will_ work uniformly across the different hardware stages, whereas .epub display is maddeningly _inconsistent_...
I would think more k-books are sold than e-books because by now most people shopping for an e-book reader know that Amazon is the leading vendor in terms of sales. I doubt the average would-be purchasers understands the differences in file formats, and the incompatible variations of DRM between the various epub vendors. Some people *do* understand that epub has the advantage of allowing them to borrow e-books from their local library. Beyond that, I suspect most buyers are strongly sensitive to price, and functionality. Amazon has done very well with their $139 price which includes wifi. Strangely, the support of epub by several vendors does not necessary come across as a plus: Apple implemented their own incompatible version, Bordors (Kobo) is imploding, B&N (Nook) isn't that financially strong either, and the Sony Ereaders, while interesting devices, have been percolating along as "also rans" for many years, without Sony ever managing to "hit one out of the ballpark." The ADE-based devices such as the Sonys run into the same problem as Adobe PDF: Long on technology, and short on just plain readability and usability. When push comes to shove, what one wants from an e-book reader is the ability to *actually* read books on it, which means it better be really good in the readability and usability department, because when reading a book, unlike using a browser, one doesn't just walk away from it after five minutes if your eyes are getting tired.