Another reason titles can appear but are not available is because they were recently withdrawn from sales. I have a book I just withdrew from Amazon sales in the past week or two, but it still appears on their website but with the message that it is not available. So for example, if an author is in the process of releasing a new edition or update, then the older edition might suddenly be pulled. It still shows up at least for a while though no longer available for purchase. I assume that those who already purchased such a book would still be able to re-download it though.