FWIW...
The D2 made a 6x7 carrier, 6x8 carrier, 6x9 carrier, and a 2.25x3.25 which were all different frames, I have them all.
A sheet of 2.25x3.25 film FALLS THROUGH the hole of the roll film holders. It took a while to find an official 2.25x3.25 Carrier, but they definitely are a separate thing.
Also keep in mind that not all photographers keep their film in STRIPS, some use individual frames, so the cutout had to have enough area to hold the frame without it falling through. This means cutting into the frame enough to avoid that, so, you lose a bit of it.
Furthermore many 35mm and other cameras' viewfinders don't actually show the full frame, there's a crop factor, so when people shoot, they might not realize there's more outside their viewable frame, so when they took their film to a printing lab to print, they would get back framed images closer to their original shot, not wider.
Those are just a few things that might help understand why they chose to cut into the frame a little bit.