my bad.

I should RTFM more often. This one says "the percentage below indicates the percentage of examinees who scored below you based on the scores of the entire GMAT testing population for the most recent three year period. The percentages below may change from year to year, and score reports will incorporate the most current data". The latest score reports use data effective October 1, 2003 and I obviously took the test before that and hence the discrepancy.