A novelist signs for ten percent of net receipts. Twice a year a statement arrives with a number on it. The author has no idea whether it is right.
They cannot see what the distributor reported, what came back unsold, or what went out as review copies.
This is rarely theft. Publishing runs on returns and the number genuinely is complicated. The complication is entirely on one side of the table.
Four thousand two hundred copies sold. Ten percent of net receipts. A figure for returns, given as a single line with no period attached to it. A deduction for a promotion the author was told about afterwards. The total, rounded, and a payment date six weeks out.
What the distributor actually reported, and when. How many copies went out as review and press copies rather than sales. Which returns came from which shipment, and whether any of them were counted twice across two periods. The retailer discount applied, which sets net receipts and therefore sets the royalty.
Ask. A polite query goes to an editor who is not in accounts, gets forwarded, and returns as a reassurance rather than a document. Audit clauses exist in most contracts and are almost never used, because the cost of an audit exceeds a mid-list author's annual royalty.
The publisher is not usually concealing anything. They are sending a summary produced by a system built for their accounts, to somebody who needs a different document entirely.
The useful thing is concrete: what your statement shows and what you wish it showed.