Imagine teaching a first grader calculus. A first grader isn't ready to learn calculus until he learns all the other basic foundations of mathamatics. You can tell him every detail about calculus, but there isn't going to be any learning or understanding.
I remember learning basic techniques as a white and blue belt. Then as I progressed, I would watch as it was taught again. Each time I learned more and I noticed that the teacher had included some new details to the technique. Details that he hadn't mentioned before and only now, after having needed that information so many times in rolling, was he sharing that new information. After I noticed this happening several times, I finally realized that it wasn't him neglecting details, but me not hearing or understanding the details. I was not ready for it. He may or may not have mentioned them, but what it was is that I wasn't ready to understand and utilize the information. When you start out, you learn a large movement, the basic mechanics, the big concept...i.e. what an armbar is - where to hold, where to put your foot, how to push and turn, swing the leg over, etc. As you progress, you add more and more details to make it better, tighter, more successful. You learn how to deal with more and different responses/defenses. And while I have heard that there are some instructors that leave out stuff on purpose, my experience has been that either I wasn't ready and didn't hear it or the instructor, only being human, may have forgotten something. At the higher levels, it's about details and there are a lot them. Which detail is important to you is an individual thing. For one person, a specific detail may be the difference between a submission and a escape while for another person, they may automatically do it without even thinking.