Why Is This Merge Behavior Different?

[Ted Stanion]

Hello All,

The following code is modified from section 6.3.5 of the LilyPond
2.10.33 manual. Can someone explain to me why the eighth note and half
note have their heads merged in the first measure while the quarter
note and the half note in the second measure do not?

Thanks,
Ted

---- Begin merge.ly ----
\paper {
  #(define dump-extents #t)

  indent = 0\mm
  line-width = 160\mm - 2.0 * 0.4\in
  ragged-right = ##t
  force-assignment = #""
  line-width = #(- line-width (* mm  3.000000))
}

\layout {

}

\relative c''
{
\new Voice << {
  c8 c4.
  \override Staff.NoteCollision #'merge-differently-headed = ##t
  c8 c4.
  \override Staff.NoteCollision #'merge-differently-headed = ##f
  c4 c4
  \override Staff.NoteCollision #'merge-differently-headed = ##t
  \override Staff.NoteCollision #'merge-differently-dotted = ##t
  c4 c4 } \\
  { c2 c2 c2 c2 } >>
--- End merge.ly ----

merge.preview.png
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