Quote Originally Posted by benjiboy View Post
Actually no, the 7% is to allow for the amount of the slide you lose when it's mounted for projection and the area that's lost in most 35mm neg carriers, because if you compose right to the edge on 100% Nikon viewfinders part of the image is otherwise lost.
This was the original intent. The increased coverage of the Nikon F series camera allowed for much more accurate framing for scientific, technical and copy work.
As benjiboy noted it's to allow for that bit of masking from the slide mount AND from a standard negative carrier.