I'm only going to theorize here, but perhaps with the 6100 chipset, they had to offload parts of the chipset to a "southbridge" to make room for the graphics subsystem inside of the northbridge. So, the nForce4 chipset normally has no Southbridge, but add in a graphics subsystem, the only way to cram it all together was to offload some other subsystems, instead.
Only theory, very well probably wrong.