Its called "Marketing 101". Imagine if Apple had 50 million phones in inventory at launch. It is unlikely they would sell out on launch day. But because Apple choses to launch with such low inventory levels, they will be sold out on launch day. Every news agency tomorrow will say Apple Iphone is hot and sold out. Apple hires PR people specifically for this purpose. If Apple had excessive inventory levels and did not sell out on launch, then they would get less publicity by the news.
Yes in theory what you say is correct but they literally were and are not able to make more iphone pluses than this. They've maxed out their production rate.