There's another explanation, which also explains why you can have a 'return' well after the 7-day grace period has expired.
Apparently you can request a new email 'account' for your Kindle. If you do, Amazon automatically 'returns' all purchases linked to the 'old' account. Then reinstates all purchases to the new account on your Kindle. To the Kindle owner, it's seamless. To an author viewing reports, they see a 'return' and a 'sale' at roughly the same time. The net result to the author is of course zero, and the Kindle owner (reader) doesn't have to pay for the books twice.
It's possible the same thing (return, and sale) happens when someone's Kindle dies during the guarantee period. A replacement Kindle is sent out with a new Kindle email account on it. All the books on the dead Kindle account are 'returned' (to prevent fraud in the even the owner doesn't return the 'dead' Kindle) and then re-instated on the replacement Kindle when the new Kindle is first registered.