[eh78ssxv2f]

The "change" that Amazon keeps can be fixed irrespective of how they distribute the money to the authors.

Lets say user pays $10/month fee. Amazon decides it wants to keep $2 and distribute $8 to authors. Now, there are different ways of accomplishing that:

(i) Distribute proportionately based on which books user downloaded. If user downloaded N books during the month, author of each of the books gets $8/N.

(ii) Distribute proportionately based on amount of time spend by user on each book. If user spent T hours in Kindle during the month, and T_1 time on one of the books, then the author of that book gets $8*T_1/ T.

There are pros and cons of both approaches in terms of fraud prevention, user engagement etc.