Remember back when Amazon pulled all of Macmillan's books because of them demanding a switch to Agency Pricing? Well, it looks like Amazon and Macmillan are going to estimate how many books an author would have sold during that time period and then provide them royalties for these books, and Amazon and Macmillan will split the cost of these royalties.
See this article for details.
Additionally, the article says that Macmillan will start paying authors 25% of net instead of 15% of list in royalties, as contractually required, and presumably 25% of net is more than 15% of list.
To me this smells like reactions to threats of lawsuits. I'm just speculating here, but I wouldn't doubt it if some author(s) threatened to sue Macmillan for the lost Amazon sales, and in return Macmillan threatened to sue Amazon. This would explain why Amazon is willing to contribute to the lost royalties.
More interesting, at least to me, is the revision to the royalties in general. I always thought that authors were getting hosed on this Agency Pricing scheme. Before agency pricing most authors with Macmillan received 15% on list price, so a hardcover with a $25 list price would be $3.75 in royalties. Under agency pricing the list price of ebooks is now set by the publisher, so now the author is getting 15% of a book that has a list price of around $15 (i.e. $2.25 in royalties). By giving the authors 25% of net it gets them back into the range they were making before. Nevertheless, I wouldn't be surprised if authors file some type of class action law suit against their publishers that switched to the agency model. I bet they have lost quite a bit in royalties this last year.