Dear Miss Snark,
How is an author (or her agent) to know how many books were sold, remaindered, at what price, et. al.? Here's a a website that gives one pause.
Cause publishers send royalty statements that say how many books were sold and in which category and at what royalty rate. They are fearsome documents.
Every good publishing contract gives writers or their representatives (ie accountant or agent) access to the publisher's books at least once a year to make sure the royalty statement is an accurate reflection of what the publishers' accounts show.
Mistakes happen all the time. ALL THE TIME.
The only question is whether it's intentional. I vote for no as the default explanation but there are times I'm wrong.
There is a company here in NYC that only does royalty reviews. They don't charge you any money up front or if they don't find any errors in your favor. They earn their entire income by taking a percentage of errors they collect for you. That says a lot. Gail is #5 on my speed dial cause when I need her, I'm too fired up and pissy to actually remember where she is in my address book.