Think about this... Why do the vending machine people visit their machines EVERY single day and fill ALL the empty spaces? It's simple, nobody is going to buy a #G-9 Snicker's Bar at 3:00 AM when they can't see it through the glass. They'll probably walk away rather than buy what they don't have a craving for.
Same goes for ammo.
A full shelf of fresh product is a profitable shelf!
In the retail world, it's called facing. You cover the gaps of stock you don't have with products you do have. Walmart either does it themselves or has the vendor do it for them for every product they have. If a slot's empty, you fill it with something else you do have.