It seems like the firmware isn't just the OS. The OS is the same for the whole world, but regarding the preproduction units, Nokia said that some lower parts of the firmware need to be flashed, too, in order to make them more equal to production units.
I have no idea what these lower parts are about, but I suspect this is where region-specific stuff like the allowed strengths of the radios are defined, or where the FM transmitter can be disabled for certain regions.