Keep in mind that, of course, all testing of little ones this age is somewhat unstable. And the KBIT2 should be considered a screening instrument, not a comprehensive measure of cognition.
Reading your post, I gather that you are not all that shocked by the verbal score (which, after all, is only one point off of your expectations, well within the standard error of measurement), but quite surprised by the nonverbal score. To answer your straightforward question, you really can't obtain additional normative data without additional testing. In order to obtain this at school district expense, you would have to present evidence of a learning challenge, and request it as an initial evaluation for special education eligibility. If you do not have IRL concerns with her development, then that would not be a realistic avenue, and you would likely have to pay out of pocket for a private evaluator.
To the larger question of whether these results accurately represent her ability, I would point you back to your own comment that she has lower math skills than her older sibling did at this age, but similar verbal skills. That is entirely consistent with having similar verbal cognition, but lower nonverbal cognition, which suggests that the test findings are generally accurate. You might not see the differences between the two children at this point, because the younger one has had limited formal schooling. If most of her academic behavior is in self-selected areas, and her day-to-day presentation (like many small children) is primarily reflective of her verbal communication and social skills, then there is no particular reason you would have had opportunity to see her areas of less strength on display, other than in the form of early math skills, which is the exact area you have named as comparatively delayed.
