Sorry for the later than usual response. We don’t check the board as often in the “off season.”
The ABPTS website gives the following explanation about scoring (but I’m guessing you have already read this since you are specifically asking a question about a score of 500).
Reporting Examination Results
NBME scores the specialist certification examinations, and candidates are issued a report that specifies their score, the passing score on the examination, and an analysis of performance according to the major competency areas tested. Although the score is based on the actual number of questions answered correctly, it is a scaled score. ABPTS requires a scaled score of 500 to pass the examination.
Based on this explanation, I’m not sure that the score of 500 represents a specific percentage. It seems like it could change each year depending on the scaling of the scores. At least this is my understanding of how it works. If someone else has more information on this, please share!