Every camera sensor & CPU design results in a base ISO rating at which the component combination operates optimally. Purely from the standpoint of lab/engineering and technical image quality analysis, ISO200 is the best setting in the D700.
The point is that equating film ISO and its impact on image quality (lower film ISO means finer grain, ergo better technical image quality which means ISO100 is better than ISO200, ISO64 is better than ISO100 and so on) with a sensor/CPU combination optimized for ISO200 is inappropriate.
I think the photography industry in general has done a poor job of lifting film ISO/digital ISO comparative ratings off our shoulders. The standards bodies have tried to do comparative ratings, but the task is impossible due to the wide variety of engineering approaches in sensor/CPU and the constantly shifting (improvements) in digital noise reduction and control.