Low ISO noise is often a result of high numbers of pixels and small sensors. The D700 has significantly bigger pixels, probably 5 times bigger (guess, could be more or less) so collect far more light, so the inter-pixel variance of photon counts will be less. The reason going to print hides the noise is pixels are mixed together, averaged out, simulating the larger pixels of full frame cameras.
This is one reason high ISO has more noise - larger inter-pixel light collection variance. That and digital/thermal noise from the electronics.




