Why is the notehead shifted?

[Peter Toye]

 In this MWE the first  a is shifted to the left of the stem but the second one isn't. OK, it's a bit cramped, but I thought that the noteheads were scaled to the size of the staff to avoid this.
 
Oddly, if the local staff size is changed to a global staff size, the anomaly disappears.
 
\version "2.19.52"

\language "english"
\score
{ \new Staff {
  <c'' e'' a''>4 <c' e' a'>  
  }
}

\layout {
  #(layout-set-staff-size 14)
}

Regards,

Peter
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www.ptoye.com