F stop not as indicated? - advice on f6-384 Rapid Aplanat No.3 Ser.D.F.8 needed

[oldparson.art]

Hi I need advice on Rapid Aplanat No.3 Ser.D.F.8 (R.O.J.A. vorm. Emil Busch, Rathenow) f 6-384
I had tested it on a large format (10x8) camera and used the widest aperture with no problem - but at the indicated f90 it way over-exposed the image (Paper negative). I tried a couple of other lenses at the same time successfully so thought I'd make some measurements of the diaphragm aperture with digital callipers and calculate the theoretical f-stop (dividing the focal length by the aperture diameter)https://www.flickr.com/gp/oldparson/ZVf1yH
As the F stop numbers increase the variance between the indicated f stop and the calculated one increases greatly.
Can you explain what is going on with this lens?
(I measured my three other lenses in the same way and had measured f stops that closely matched the indicated value.)
With thanks
Simon