If the number in your notebook and the number on the shelf are always a little off, the problem usually isn't theft or carelessness — it's timing. A sale gets rung up, but the stock count gets updated later, by hand, if at all. Every gap between the sale and the update is a chance for the two numbers to drift apart.
The fix isn't stricter counting — it's removing the manual step. When a sale, a purchase receipt, or a return updates stock automatically, at the moment it happens, there's no window for drift to creep in. The count on the screen is the count on the shelf, because nothing else ever touches it.
This is also why a return that doesn't put stock back is worse than no return process at all: it looks like the item vanished. A correct return has to restore inventory the same way a sale removes it — automatically, not as a follow-up task someone might forget.