Incorrect letter spacing in header

[Steven Smith]

I am working on an arrangement for band, and would like the header 
information to be the same size on the conductor's score and the 
instrumental parts. I put the scores in different \book block so I could 
use the #(set-global-staff-size 12.5) for the conductor's score and 
#(set-global-staff-size 20) for the parts.

Then I tried to use    \abs-fontsize #20  for the Title in a 
\bookTitleMarkup block. The second book has the correct font size, but 
the letter spacing is wrong. If the first book has the smaller staff 
size, the second book title spacing is too wide. If the first book has 
the larger staff size, the second book title spacing is too narrow.

What is going on here? MWE code follows, images attached.

-Steven Smith

-----------------------------------------

\version "2.19.80"
\language "english"

notes = \relative c' { c1 d e f g}

% Just to make the image smaller
#(set! paper-alist (cons '("my size" . (cons (* 8.5 in) (* 2 in))) 
paper-alist))

\header {
  title = "Adios, Au Revoir, Auf Wiedersehen"
  tagline = ""
}

\paper {
  #(set-paper-size "my size")
  bookTitleMarkup = \markup {
    \fill-line  {
        \abs-fontsize #20
        { \fromproperty #'header:title}
    }
  }
}


#(set-global-staff-size 12.5)
% for conductor's score
\book {
  \score {
    <<
      {\notes \notes \notes}
    >>
  }
}


#(set-global-staff-size 20)
% for Instrumental parts
\book {
  \score {
    <<
      {\notes \notes \notes}
    >>
  }
}

headertest.png
Description: PNG image

headertest-1.png
Description: PNG image