At the wider end, there is
considerable barrel distortion. Probably an optical trick to reduce corner fall-off as the Nikon software automatically corrects for the distortion, likewise when jpgs are generated in-camera. You do lose a tad of the wide coverage, but unless you study uncorrected NEFs against corrected files (NEF/jpg) the reduced angle of view is hardly noticeable. Plus, not all subjects have straight lines to demonstrate the existence of the barrel distortion anyway.
In this case, the floor curves towards me and the ceiling isn't perfectly flat. Thus I ran the NEF without any geometric correction through Photo Ninja. Seeing the image again now, I'm reminded to buy a new 'Kånken' backpack as my old one of 30+ years has become "a little worn"
(hint: the leftmost girl carries this backpack)