It could happen for many different reasons - one very simple thing might be that she was answering on a bubble sheet and accidentally skipped a line, and then all the answers after that were wrong just because she wasn't putting the answer on the right line. Sometimes kids rush through tests quickly and don't check their work. Maybe the math section was late in the day or right before lunch and she was hungry and just not interested. Maybe she works more slowly at math and got every problem she worked right but didn't have time to finish. Maybe she is a very very smart girl who's strengths are in places other than math. Maybe her school curriculum didn't cover what is covered by most children in her grade level in math so she doesn't know how to answer some of the questions.
I wouldn't put too much meaning into one lone outlying score. I would ask to see the full test report if you can, so that you can gather any knowledge that's available - ask if you can see how many problems attempted vs how many correct etc. I would also ask your dd if she remembers anything in particular about that specific part of the test. And then I wouldn't think about it anymore unless either:
a) She needs higher math scores to get into a program you want her in. If that's an issue, show that the math score is anomalous and ask that she be tested again or ask that she be allowed in based on math grades or examples of her math work etc.
b) If she seems to be struggling in math or isn't getting good grades in math in school. I suspect this isn't the case, but if it is, it's worth looking into further.
Best wishes,
polarbear
