Your software doesn't do metric
I used to be support manager for a company that supplies CAD/CAM software to the education sector. Rather than provide complicated 'locale' settings the developers had decided to have the application check the Windows settings and used the preferred units set there. We'd regularly have teachers berate us for using inches when 'anyone should know that schools have used metric in D&T lessons for years'. A surprising number of school PCs were set to default to inches. Probably still are.