Yes - it's hard to appreciate how far camera tech has come since the 40s, and presumably also a typical soldier isn't bringing a high-end 35mm camera into the field.
More specifically, if there's play in the gears of the winding mechanism then you'll get a lot of variance when it comes to inter-frame space.
> "I have doubts about this statement. Can someone with more expertise in this matter explain?"
It doesn't have to be nefarious - they can assert copyright over the scans of the film without asserting copyright over the original images themselves (though in this case it does look like if you donate film they want the whole shebang).
It's like recording a cover - your recording itself has a separate copyright regardless of the copyright status of the song upon which it is based.
Like a cover song there could be some legal issues between the scanner and the copyright owner of the original film images, but presumably they sort this out between themselves.