Most Walmarts never have all calibers and each store seems to have a different supply of what they do have on hand.
You guys are looking WAY too much into this. What probably happened is that someone bribed one or more of the staff to hold the ammo for them. The person who works when they get shipments in sets the ammo aside and gives the person a call.
This does happen and local retailers do sometimes buy what they can. There is a slim margin on ammo and most gun shops are not given any prior notice of expected ETA for ammo shipments from their distributors.
This is part of the reason Walmart temporarily started limiting customers to six boxes.