[Chrysalis]

An explanation I can offer is one of two things.

1 - seller has multiple shop fronts, and the stock is shared across them all, if they got too many orders across all the shop fronts then they have to disappoint some buyers, I had a seller confirm this happened to me when he said he sold too many across amazon and ebay to fulfill orders.
2 - Some retailers advertise stock of the wholesaler instead of their own stock, so you place an order, and then they order from the wholesaler (this is also why sometimes delivery times can be quite long), sometimes the wholesaler doesnt actually have the stock so again the seller fails to fulfill.