I just thought of another possibility, which is that the NPA (percentile by age) and NPG (percentile by grade) may be slightly different, depending on when in the year your child's birthdate falls. It is possible that the principal gave you the percentile for one, and the standard score for the other.

This ppt is one of the training documents used by the publishers (this one happens to be for ACSI):

https://safe.acsi.org/iweb/upload/PPT_InView-PTCS%205_17_Ppt.pdf

Unfortunately, I couldn't find a document directly from CTB that would provide the percentile-standard score correspondences. I calculated the standard scores based on my prior knowledge of statistics and easy access to percentile-z score tables.

...pronounced like the long vowel and first letter of the alphabet...