Is this someone opinion? Why is it deliberate?
I assume you are asking this question in seriousness, so I thought I would answer. I am not an Apple employee "in the know" and Apple is not one of my company's (direct) competitors, so these are educated guesses. But I think they are good guesses.
One reason falls under the category of competitiveness. If your competitors, known and unknown, do not truly understand how fast you are turning around specific products at specific prices, it can make it really hard for them to compete by undercutting you. A second reason is vendor/supplier channel . . . management.

If no one on the outside knows exactly how many widgets you will need, then you can better play vendors/suppliers against each other. A third reason is what some might call accounting and/or marketing games. If you do not report a lot of specifics, especially if you are still making a lot of money for your investors, they will let you get away with some vague reporting in some ways . .. and when the product mix has to change later, you do not get lots of annoying questions about the success of this or of that. You can just say as a whole this chunk of product category is doing great and deal with under-performing lines exclusively internally.
A fourth reason is just my judgment watching this company over time. They are just so secretive about locks of things--some for reasons I have just written--that I think it is second nature to them.
I hold a pretty senior position at a public company, and reasons 1, 2, and 3 are business legitimate reasons we consider all the time. In fact, I help craft a fair part of our strategies and they can center on such reasons. And this is coming from someone with a lot of science/engineering in their background, so you would definitely be correct in guessing I prefer having data. Lots of it. But I realized long ago that I am not the audience for lots of stuff, and that you can definitely get to where you have to write off a whole product cycle if you not at least consider and address such things. Maybe if your widgets are always the best for everyone at a price everyone loves and still make you crazy amounts of money . . . maybe then you could be fully transparent. Probably not, though. And I have never worked for such a company, certainly.