I believe the reason why teachers at AS/A level don't like to go into much detail, other than sticking to a syllabus, is because it as mentioned in the above posts can confuse students. Let me give you my case. At IGCSE, in Maths I was taught differentiation. Just the basic dy/dx. I had absolutely no clue why it was done. Then when at AS level they went through differentiating through first principles, I just thought, WTF is this! To be honest, I still can't do differentiationg "through first principles", and don't think i'll ever be able to because it's confusing. Little by little, i started getting the grasp of it when i understood the basic, but then so many questions came into my head that I just thought of just absorbing it, which usually isn't advisable at AS level physics. When I went onto C2, I understood the main point and now, I can do it perfectly understanding what the second derivative is used. This is a simple case, if you think about it, but in Physics, it gets a lot more difficult.
In the kinetic theory, where you relate equations which pressure, volume and temperature, when I got into the class, the teacher had already put the formula up and stated the symbols' meanings. I didn't understand a word of it. Then I went up to the teacher and he explained me everything. It still seemed like he was talking Japanese as the theory is made up of assumptions and nothing made sense. Then I bought this book, and it went through a mathematical way of explaining it, and yes, it made a little bit of sense but was difficult to understand all concepts. After having read the mathematical way of getting the Pv=......, I wasn't surprised why the teacher didn't explain it.
Basically what I'm trying to say is what teachers do, may be a "fake" model, if you want to say, to explain things so that we understand the main concept of it. When they go into detail, that's where the clever and bright students absorb the point of it and other don't. That's what my point of view is.