It appears that the world mag model in LibrePilot is out of date. New data is released by NOAA about every 5 years. The data in flight/libraries/WorldMagModel.c expired at the end of 2014. The new one was issued by NOAA on 15 Dec 2014. The official download url is in the .c file.
Since the NOAA distribution format is different from what we use it would probably be easiest to copy the table from TauLabs. They moved it from the .c file to shared/api/physical_constants.h