The difference between one hourly census and the next, for 15 rares and nothing else. An item entering the game was not in anybody's save file an hour ago and is now; one leaving was and is not.
A trade moves an item between two saves and appears here as nothing at all, which is the point: this is not a record of who has what, it is a record of how many exist.
How this is counted, and what it cannot show, is on the notes page.