I have just found this statement in Wildi (4th, revised, p.125):
"The image seen on the ground glass of the Hasselblad camera, or the groundglass back when attached to the rear of the camera, corresponds to 91% of the area covered on the film with the waist-level finders, and 88% of the area for the PM5 versions of the prism finder. This smaller area coverage is done purposely, as the slide mount covers up a small area of the image on the film."
This is considerably less than 100%, and I am rather surprised, and shocked, to this out.