I don't know much about Lexile scores, but I do give reading assessments (we end up with a guided reading level instead of Lexile). Obviously I don't know your DD at all, but I can tell you two factors that sometimes contribute to a lower reading level than expected.
First of all, some teachers do not go above a certain level for a certain grade. I know, I know--but you do get those rigid ones who just say, "This is as far as I'm going. Further will not give me any more information." Sometimes that is true. For instance, I have a 10 year old in my classroom who tried to read The Bourne Identity on Friday. That was a big fat NO. Regardless of his comprehension, it is inappropriate for the age level.
Secondly, comprehension is not just about summary and retell. A student needs the whole package to really be comprehending what they are reading. That means the ability to analyze character (what kind of person is this character? how can you tell? can you prove each character trait?), the ability to track character change across a whole text, the ability to make inferences about character motivations, etc. There are many more higher level reading skills--these are just a sampling.
Sigh. Finally, it is possible that your DD is giving answers that are not traditional "correct" answers, and the teacher is not recognizing that your DD's answers are equally valid. I had a similar thing happen to my DD, when she was tested on a story about a girl and her cat. When the teacher asked, "What lesson did you learn from this story?", my DD answered with a lesson she had learned from the CAT, not the little girl. The question didn't specify where the lesson should have come from, and I would have accepted the answer--but it wasn't traditionally "correct."
If the reading level doesn't overly limit her choices in the classroom, I wouldn't give it a second thought. It's just one person's snapshot.
Sorry to write a novel. I just answered a whole bunch of emails from parents of kids in my class, and the explanations were flowing.