the new vertical spacing between systems syntax

[ivan . k . kuznetsov]

Concerning the examples in the manual, section 4.4.2:

   4.4.2 Explicit staff and system positioning
   
http://lilypond.org/doc/v2.14/Documentation/notation/explicit-staff-and-system-positioning


I don't understand the reason that the variables that
override default spacing between systems:

  #'line-break-system-details #'((X-offset . 20))
  #'line-break-system-details #'((Y-offset . 40))
  #'line-break-system-details #'((X-offset . 20) (Y-offset . 40))
  #'line-break-system-details #'((alignment-distances . (15)))
  #'line-break-system-details #'((X-offset . 20) (Y-offset . 40) 
(alignment-distances . (15)))


are within  one of the "\new Voice" brackets:

        \new Voice { }


Since these vertical spacing over-rides are to
supposed to effect spacing "between systems"
why are is this syntax inserted within 
just one of the voices of a system?

I would have thought that syntax to indicate
spacing between systems would have been
placed within the "\score {}" brackets before
any of the "\new Staff" brackets.


Is my question clear?  Thank you for your help.