the f number of a lens is a function. That function is defined as the focal length of a lens divided by the diameter of the entrance objective. It is the focal length that causes the confusion when comparing lenses. Focal length is defined as the distance from the lens nodal point to the focus point (film or viewing plane.)
It's the lens nodal point that causes the confusion. For some lenses it is at or in front of the iris. For others it is behind. In other cases it can be in front of the entrance objective (I think this is the case for the Nikkor UW lenses). The lens design determines this.
Standard, M2, M3, IIS2, IIIa, Signet 35, IIIC, Ia ...