Its all about statistics really. I work in the quality department at one of the Cummins engine plants and its simple PPM (parts per million) just like we have here out of every so many orders, one will have a flaw. So when you think of the big picture yes eventually you will get an order thats wrong. However to them for that week you could be order number 251,765 so in the world of processing and shipping thousands of orders a week thats really not that bad. At the part store they dont have to worry about that. Computer says its this part number, if its wrong then its the computers fault.