Because D4 DX just cropped away most of your pixels. DX mode is only 1.5 crop. So FF had 2.25 more area and pixels.
My thought was that having relatively big pixel pitch DR is less affected by the area light is projected to, and 33% less area doesn't make such a big difference.
I've checked the web-site again and noticed that the (e) prefixes disappeared from the D4 labels and the difference between DR of those became closer to 0.6 stops.