Each sensor has a base ISO - apparently, it's 100 for some, 160 or 200 for others. Anything up or down from there is gain applied to the signal, analog gain (or negative analog gain, i.e. attenuation) for the 'native' ISO range, digital gain (loss) for the expanded ISO settings (L, H1, etc.).

Going lower than the base ISO doesn't really deliver better IQ. The problem with designing a sensor with a lower base ISO (which would have better IQ at that lower base) is that to achieve the higher ISO settings, more amplification would be needed - and that means worse IQ. The industry is trending to higher ISOs, so I don't really expect much of a push downward.