Display of notes when using reduced line count in staff

[Andrew Bernard]

Pardon my base ignorance, but when using a reduced line count of 2
lines for a treble staff instead of five, the middle C position
apparently moves. I can't quite figure this out. I suppose the
behavior is entirely obvious to some, but I can't get my head around
it. There is very little discussion of this in the archives, if any.
Can somebody help me understand this? I would have expected the notes
to stay on their lines and spaces, but there is obviously more going
on than I thought.

[Readers of the list will realize this is related to the setting of
Ambrosian chant just raised in a post.]

Andrew

%====
\version "2.19.20"

music = {
  \key f \major
  bes'4 c''
}

\score {
  \new Staff \with {
    %\override StaffSymbol.line-count = #2
  }
  {
    \music
  }

  \layout {
  }
}

\score {
  \new Staff \with {
    \override StaffSymbol.line-count = #2
  }
  {
    \music
  }

  \layout {
  }
}

%====