Not a chance Apple is throttling delivery. Apple has to decide months in advance what the launch quantity is going to be, based upon previous year's sales quantities, anticipated demand and the other important piece - production capacity. They also have to figure out what they can produce and deliver week by week over the course of the next 90 days, or whatever the point is they estimate demand plateauing at. It would be a simple thing, if they offered just one size/color/capacity/cellular chip, but of course that isn't how it is, so they have to juggle multiple iterations across a global market.