Think about when you're metering with a view camera. You're metering the light of the scene with no "attenuation" whatsoever. Your aperture on your lens however, will of course choke out some of the light as it makes its way to the film. So we tell the f/stop to our meter to account for this.

But if we're already inside the camera, there's nothing that's going to attenuate the light further at this point. By using f/1.0 in the expsoure calculation we are effectively eliminating this altogether, putting 1 in the denominator.

This meter goes down to EV -2, which I think should be plenty to measure daylight pouring through an f/9 aperture.