note column and dots

[James Bailey]

Why does the dot move in this example?
\new Staff <<
   {
      \voiceOne
      \once \override NoteColumn #'force-hshift = #3
      a8 g a2.
   }
   \new Voice {
      \voiceTwo
      a2. a4
   }
>>

Curiously, it doesn't move in this example:
\new Staff <<
   {
      \voiceTwo
      \once \override NoteColumn #'force-hshift = #3
      a8 g a2.
   }
   \new Voice {
      \voiceOne
      a2. a4
   }
>>