I need a geologist to explain this one. Moissanite has a Mohs hardness of 9.5. I guess it is easier to scratch than a diamond, but the scratability should be indistinguishable between the two for all practical purposes.

Not a geologist but Mohs hardness is an ordinal scale so the distance between 10 and 9 isn't well-defined. The numbers are defined as being specific minerals.

Diamond (10) is 4x as hard as corundum (9) which is 2x as hard as topaz (8).

https://en.wikipedia.org/wiki/Mohs_scale#Reference_minerals