I think it’s being used pretty consistently. It’s about openness with and about game mechanics.
But if the game expects GMs and players to make sausage themselves, then I don’t get the resistance to the idea.
I'm saying it's not a particularly realistic expectation. There's a difference. They have to take into account page count and effort; for something like the detail you seem to be advocating that would require a lot of work. Besides that what, exactly, would need that detail? At some level every paragraph in the book has some thought put behind it. I could see a separate "Making of" book might be of interest but I don't think it makes sense to expect much in the core book. If you want to know the theory behind writing an RPG, there are a lot of resources out there.
Everyone knows the DMG needs an upgrade so we'll see what we get later this year, but for some people no reasonable amount of explanation for people would be enough. For that matter, most people don't care. It's just not going to interest most people playing D&D. I don't need to know the theory behind why they made bows dex based in order to have a house rule to make them finesse so strength based PCs have a decent ranged attack option.