DD8 is a bit impatient with answering questions, too, but I see it primarily with homework worksheets. She has a "rush and get through it" approach that makes me crazy! I find sloppy, silly mistakes way too often!
With DD (and this could be your DD, too), she wants to get it done quickly so she can go do something else. She does not seem plagued by perfectionism. I am not sure that she finds grades particularly motivating either, although I do think she likes to do well overall. DD learns things because she finds them interesting...and well, some things apparently seem less interesting to her (like worksheets).

Could your DD try to think of the assessments like she does the other testing that she does well on? I am surprised that the school would completely ignore the standardized testing (since schools these days seem to love tests so much

).
Although my DD does not like worksheets, she does very well on testing.
I agree with KADmom - test taking skills are still developing and if your DD has strong measures of performance, she should not be underchallenged because of a single measure.