I had wondered for a while how the credit system worked, and if the increase in sales made up for the 'deal' off audiobooks. I get $50-$60 audiobooks for $15, there's something mismatched there. Either the audiobook was priced at an extreme premium, or, it seems, Amazon just isn't paying the price I would expect.
Edit: I found one response from an author that I think makes a simple point, if not perhaps what the thread describes (still can't tell). It's impossible for authors to calculate their expected royalties as there are so many varieties of discounts and price points that customers can pay for a single book: https://twitter.com/AdamEcclesBooks/status/14933236569105489...
Which makes sense to me, as there's purchases and credits and various subscription tiers and audible unlimited and these are just the things I can think up off the top of my head. But again, I would buy a small fraction of the audiobooks I have purchased at full price.