Another possibility is the teacher didn't grade it at all. When my kids were in elementary school a lot of HOMEWORK (not tests) were graded by volunteer parents who grade off a key. And are looking for very specific answers and don't know what a teacher wants in this kind of question. I remember being told to grade by looking for specific "key" words.

Good luck. We have this kind of problem in my house with my DS15 and I'm still working on his answering what the teacher WANTS. He takes questions a bit too literally and then gets frustrated because the 'right' answer doesn't really answer the question in his eyes.