Returns make shipped and sold different numbers
A book on a shelf is not a book sold, and the adjustment can arrive two reporting periods later.
The publisher receives sales reports, nets off returns, applies the contract, and produces a statement. Each step is legitimate work that the author never sees.
A book on a shelf is not a book sold, and the adjustment can arrive two reporting periods later.
By the time it reaches the author, several rounds of aggregation have happened, none of them visible.
Most contracts permit an audit. Almost no author can afford one, so the right is theoretical.
A publisher acting entirely properly still cannot demonstrate it, which poisons relationships that both sides want to keep.
The useful thing is concrete: what your statement shows and what you wish it showed.