Um, because you're not an NT.
If I knew, then I'd basically be NT.
Anyway, I often have the same problem; whenever i read something that doesn't interest me, I have a really hard time actually absorbing the information, and understanding the meaning behind it. I had this problem with my high school textbooks, and I also have this problem with the learner's license manuals. If I didn't have this problem, I'd be done high school and have a drivers license by now.
My advice is to read through things and not care if you understand them or not. You can re-read it later and you'll probably understand it better. I did this with my learners and I passed on the first try. Even though I felt like I couldn't understand it.