as to why there are different base iso, i don't know. the d3 of nikon also has base iso200.
the old k100d also had base iso200.
essentially, what is done to achieve iso100, is the camera exposes and then dials down the exposure. i know doing this loses some headroom in the headlights, based on what happens when you set iso 50 for canon 5d (base iso100), and that makes sense... but so far the measurable DR of the k-r seems to be greater at iso100, don't know why - not a techie.