Grit (both CAMI amd FEPA) is based on the size of the grains that pass through a series of increasingly fine mesh sieves, with the numbers representing how many holes per a given area there are in the sieve. However, the size of those holes depends on both the density of the wires and the thickness of the wires (thicker wires = smaller holes if you have the same density of holes), so I would imagine that the two standards diverge because of that. I don't know for sure though, and those standards are expensive to buy

I don't think you could use SI prefixes with the grit systems, but ultra-fine sandpaper often abandons the grit systems and just gives you an average size of grain in micrometres