Re-projection from Lat/Long to OSB36 looks distorted and does not match Ordance Survey tools

I'm re-projecting GPS points  from ETRS89 to OSGB36. The difference in the re-projected points between the Ordance Survey tool (Grid Inquest II) and Global Mapper 18 is slightly mor ethan a metre. Is this because of the grid file used in Global Mapper, and/or does the Global Mapper re-projection tool support ISTN/OSGM15?
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