One of the distinctive aspects of ebook publishing is the willingness of authors and publishers to vary book pricing. Some of this may be an attempt to find the ideal price for a given book, but others are following a more sophisticated playbook, a little like the strategies employed by airlines with ticket sales. Kindle itself has Kindle Countdown Deals, though there are built in limits to the use of this particular tool. PublishDrive makes the following observation:
And WriteHacked has a long post on the topic spelling out all of the options and reasoning behind them