Ok that makes sense - it's a function of the hardware light sensitivity (I.e, how fast light fills photosites) of the sensor itself, so if a sensor's base ISO is 200 and you shoot at 100, the camera has to de-amplify the analog data of the actual photons received at the photosites.
So shooting below base ISO doesn't make a lot of sense, then, in terms of IQ, and possibly DR, because you might clip highlights sooner at an ISO that's less than base. But in practice, perhaps this is not noticeable.
Thanks - you've pointed me in the right direction to do some more research.
Found an interesting discussion or two here:
Which led me here:
I am a software guy so I love the technical details. I also have a need to better understand sensor tech, as well as LCD tech, for a project I'm working on, not photography related.