If the online inventory checker works like Best Buy's, they may be in-stock, though not in enough of a quantity to guarantee someone will be able to pick it up in-store.
BB's online inventory status shows it's in-stock when there's 3 or more of the item on-hand and not in-stock when there's 0-2 of them on-hand. So, if the website shows it's not in-stock, it might still be in-stock. Having BB check in-store via RSS will show a more accurate count.
Sears' inventory system for in-stock checking has been reasonable accurate from the times I've used it, though I don't know if they have that "fudge factor" for out-of-stock notification that BB does.