Bob no component is perfect, I suppose they are covering the possibility of some variation at different points on the wheel due to run-out, etc. As you observe, this should be tiny IF the components are good and IF they are correctly assembled... No point getting it all set up lovely at 12 o'clock if there's a tight spot at 6 o'clock.
My own experience is limited but I found it all much less clear than pictures in books!
My own experience is limited but I found it all much less clear than pictures in books!