Eh, it doesn't matter. Apple going to sell 100 million of these things between now and Christmas. Next year, same thing. They simply can't make enough of them to have on hand at launch (or for a few weeks after). This is nothing new, and has been ongoing since the 4 came out.
No other phone vendor has the same issue, because to them, if they sell a million in a week, they are giddy. Apple sold 34,000 phones per hour for an entire financial quarter when the 6 came out. I don't think for a moment Apple is holding back; I do think there simply isn't enough manufacturing capacity to produce what Apple needs at times.