The count is always wrong.
It's continually updated, but it's still always wrong. Because the amount of labor cost to ensure it's accurate at all times would be a silly use of scarce resources.
Sometimes it's as simple as a customer picking up an item, wandering around, and dropping it somewhere else. Sometimes it's theft. Sometimes it's an incorrect count in receiving.
All of this is an average day at a warehouse store. Walmart deals with the same thing.