Apple products are shipped the same day, unless they are Build-To-Order or in high demand.
Apple is a completely different business than DELL. Apple sells in its own retail stores, through big box retailers, through Apple Service Providers, and via its own web site.
The high-demand products must be experiencing some kind of limit in manufacturing. It could be the factory producing the screens can only make so many. Acquiring the components is not a trivial thing with products that aren't cookie cutter.
It's not a matter of demand. It's a matter of acquiring the components, building the product to spec, testing it, and preparing it for shipment.