I read here that:
Apple began iPhone 6S and iPhone 6S Plus mass production in mid August.
I assume that's somewhere at the end of manufacturing process they need to install operating system on the device. It's widely anticipated that new iPhones will come out with iOS9 preinstalled. How can Apple install that new OS on those devices manufactured in August if iOS9 is still not ready yet? Or maybe iOS9 is already finalized and they are just not letting us upgrade older devices to it yet? Or maybe somewhere, as a part of the activation process, a new iPhone downloads patches for the not-fully completed iOS version that was preinstalled on the device?
PS I realize that this is not unique to iPhone 6S and iOS9 and it must happen every year with a new device and new iOS version.