There are many variables to consider. The best way I can describe it is this:
Each Retailer, like Sprint, AT&T, Target, Best Buy, etc, have 2 purchase methods. Online and In Store.
Each retailer has 2 shipment inventories. Shipment A goes to their stores for store purchase and shipment B goes to their online order shipping warehouse where online orders for fulfilled. Now, I would assume with the carriers as they were offering online promos only like Sprint was, including pre-orders, most of their orders were received online. So their warehouse shipments have stayed under high demand which is what is causing delays in shipment. They have the largest amount of orders to fill. Meanwhile, the actual stores that get shipment B are experiencing not as heavy demand for in store purchases so they tend to have devices more readily available. Example, Sprint may be sold out online and backordered for say the 7 Plus in Jet Black 128GB, but there may be some of those available in store, people just aren't going there to look for them, resulting in ordering online to save money. And most people prefer to order from either Apple or their carrier, which is why it's been told that it's been a lot easier to walk into Best Buy or Target and actually find these impossible to find devices available right there. Lower demand. More devices available. Non of this is exact science. But there's more to it than someone got the same phone as me and they ordered after me. The retailer, the method they ordered, it all plays a factor.
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