I would like to politely inquire where people heard about the notion of some iso being real, and some being "done in software". I am just asking, because I never heard of it, and my notion of how ISO sensitivity works for digital suggests that the amount of gain from the sensor is selectable by say a voltage level, which can be any value between the low and high values. Therefore the ISO values like 100, 200, 400, 800, 1600 are just arbitrary as far as the camera is concerned, they are values that are meaningful to us as humans. The gain could be set to something which equals ISO 50, or ISO 25600, whatever the sensor gain can be set to.
I'm certain someone else can explain it better.