header anomalies LP2.13.60

[Bill Mooney]

Greetings!
Can someone please explain what is happening with the header block in 
this code?
+++++++++++++++
\version "2.13.60"
  \header {
    title = \markup \center-column { \huge "XXXXXXXXXXX" "XXXXXXXXX" 
"-X-" }
    subtitle =\markup \center-column { "NNNN" "-N-" }
    subsubtitle = \markup \center-column { "ZZZZ" }
  }
\score { { a' b' c'' d'' } \layout { } }
+++++++++++++++
The first line of Xs is rendered smaller than the second line, and the 
-X- and the -N- are too close to the lines below them.
Is this behaviour due to something I've misunderstood, or is it evidence 
of a bug?

Many thanks,
Bill