Very bad analogy, the calculation in metric is correct unlike the calculation with furlongs, that's the whole point.

    660 ft/furlong * 8 furlongs/mile = 5280 feet/mile != 5000 feet/mile

    1 mm * 1 hectare = 0.001 m * 100 m * 100 m = 10 m^3 = 10 000 dm^3 = 10 000 l