I don't have much insight into the tests and I don't trust grades much either. You can see your child and you know what he is capable of, thats all I can say.
About the reading part, when you say he is reading at 6th grade level, do you mean he is reading the books aimed for 6th grade or has he been administered some 6th grade level comprehension tests or exposed to 6th grade level comprehension work and did good. Did you try the comprehension tests at his level and or above level? If not, try those at home and observe him. What issues is he having while doing that work? Is he not paying attention, or having vision issues or having a difficult time following through - make him talk about the challenges he is having when doing that work.
Also many times, a small discussion every now and then about what they are reading can give an idea about how much they are comprehending.
