Several surface treatments or production methods have the effect to compress materials and make them more dense than their natural state or create changes in the way their structure is disposed. This in turn causes changes in the scratch resistance so the Mohs scale alone is just an indication.
Anglage made with old manual techniques for example has not only an effect on the visual side but also compacts the surface of the metal making it harder. Machining them and removing material not only has a worse effect on the final result, also does not have this advantage.