Bogbeagle... as you may have guessed the design of the procedure assumes a level of accuracy from the pilot, and although that accuracy will be included within the original designation of the Final Approach Segment Size... the specifics are not available to a procedure designer... and I believe you should be asking that to a pilot Instrument Rating examiner not an ATCO/Procedure Designer.
What I can assure you is that, as DFC says, the segement takes into account aid accuracy and error, pilot accuracy (whatever that is) a whole host of other factors and then a bit for the wife & kids for good measure. Look at the figuyres above, the the segments for NP Approaches are massive... hence high minimas often have to be accepted.