I was told that every time an item is scanned at the register at any Walmart store, it updates an inventory database in near real-time (I assume there is some sort of message queue with that kind of volume and distribution). Then their system automatically places replenishment orders with their vendors when the inventory of that item goes below a certain level. They are a well-oiled machine, focused on making sure that they almost never lose any sales due to being out-of-stock, nor waste money on carrying excess inventory in their warehouses. Mismatching inventory to sales will drive a retailer out of business very quickly.

This is also part of the reason why they are able to push "everyday low prices" instead of discounts. Discounting is what you do with the excess inventory when you messed up and ordered too much of something.