Amazon has changed their TOS to allow them to price match digital books to the cheapest PHYSICAL price as well as digital books on other sites. I became aware of this when several of my books' prices were mysteriously price matched to less than 50% of their list price. They were all books that had been up for years without any price changes or free promotions, and I couldn't find the books on sale anywhere else at other than the normal list price. After over a month of KDP support's usual "we need time to figure it out" I finally realized that they were price matching to the used paperback prices on Amazon.com. I just checked their TOS and found this:
The price at which we sell your Digital Book may not be the same as your List Price. For instance, we might sell your Digital Book at a lower price to match a third party's price for a Digital or Physical edition of the book. Or, we might match Amazon's price for a Physical edition of the book.
In addition, they have changed the TOS regarding author royalties on books that are price matched:
If we price-match your Digital Book, your Royalty will be:
The Royalty Rate indicated above, multiplied by the price at which we sell the Digital Book, less taxes and Delivery Costs, for sales to customers in the Available Sales Territories.
So they are no longer paying based on the list price if they price match. This means that if you have physical editions of your book available, you are probably getting screwed.