A problem with DSLR cameras is that sometimes they have too many controls and options.
It is such a new camera on the market, I doubt that there are many people who have run into the situation you have encountered.
I checked the User's Manual and found two possibilities:
Check Pages 37 and 76 of the User's Manual.
Either one, will cause the problem you are seeing.
If that does not solve the problem, you should contact Nikon and talk to one of their customer support people. They are more familiar with that camera.
I recently purchased a Nikon D40. I have it in manual mode, with ISO set to (for example) 400. When I go to take the picture, the camera changes the ISO by itself, and when I display the information on the picture, the ISO is typed in red. The ISO changes automatically to the correct direction. For example, too bright, it decreases the ISO, too dark, it increases the ISO. Even in manual mode, it rarely uses the ISO setting that I tell it to use. I'm not very experienced with knowing what apertures and shutter speed to use in what condition so I don't know if I'm under or overexposing unless the camera tells me. So why does my camera use a different ISO than it's told? How do I fix this? Thank you in advance.