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Eeyorecore
That's already half the error you originally claimed, and just 1mm different from what I would predict if you asked me to guess at a starting point for substituting JIS for ISO.
If you look at all the potential sources of axial displacement error at the pedal end of the crank which are generated by the boss/spindle taper interface, you'll quickly realise that you can tell nothing about the "standards" from assembling one crank on two BBs, especially if you are just eye-balling it rather than taking proper measurements with the cranks in all 4 possible orientations on the spindle and with consistent bolt tension. How precise is your torque wrench?
If you wan't to reverse-engineer the standards, you should probably take at least 10 samples each from 10 manufacturers (5 JIS, 5 ISO) into whichever room your CMM is kept in, but my previous advice stands: check every individual installation for correct fit, because wear and tolerance stack happens. Either that or ditch the stupid things and get some cranks with a proper spindle attachment.