The iPhone uses NAND flash memory, which both Samsung Electronics, the world's No. 1 flash memory maker, and Hynix Semiconductor, the industry's 3rd biggest, manufacture. Both companies sell their NAND chips to Apple.
The iPhone's performance in Asia has been modest compared to the US and elsewhere. Hindrances like a lack of Wi-Fi and a strong black market have affected sales in China. Eager crowds welcomed this week's release in Korea, but in Japan sales have been quite slow.
I'm no market expert, but I imagine Apple must predict how many units it needs and doesn't always hit the mark with 100% accuracy. We'll follow this story as it develops.