That does make sense.....
So rather than turn down a few meters of Alum. bar down to 16.5mm (or be "true" on all spurs...)- they just start at 18mm for everything and adjust the tooth cut accordingly... It's probably much cheaper to do that way... Then just play with 2-3 tooth cutters until it works.
Diverting from true pitch doesn't make sense to me - but thats more my logic and background I guess...... Good thing I've only tried to make the 36T spur so far! I would have wasted yards of rod! (and I still may....)