It’s Greeks vs Romans.

Western finance/accounting industries adopted ‘M’ from the Roman numeral for 1,000 to mean “thousand”. MM (or mm) meaning “thousand thousand”, or a million.

Separately, when the French invented the metric system they used the Greek prefixes for multiples (kilo, derived from the Greek for “thousand”, being the best known). Which is why ‘k’ denotes thousands in most other industries.

Conversely, the metric system used Roman prefixes for submultiples, which is where centi- (same root as Centurion) and milli- come from.