1DX is nearly 6 months old. OM-D EM5 is older, so is the XPro1. None of these have been tested, and yet the D600 is tested and reviews written what, a week after announcement?
Now, there's a lot of people who claim that Nikon and DXO have secret dealings, biased fanbois, in each others' pockets, whatever.
I'm not going to join them, nor am I going to dispute any of the results. (Although I do wonder, how the D600 and 5D3 have 'SNR' lines almost as close to be touching, in both print and screen, and yet the D600 is 'usable' to 2980 and the 5D3 is only 'usable' to 2293)
But like any review, these data have to be taken with a grain of salt.
And with DXO, that grain is the size of a the pyramids and growing...
*wanders off to keep shooting with 5EV DR film...*