center-column interacts with fill-line

[Francisco Vila]

Hello,
I'd like to know why is this markup centered on the left edge, rather
than on the center of the string.

\markup{
        \fill-line { \center-column {  wwwwwwwwwwwwww }  }
}

By modifying it in any of this ways A) or B) it becomes centered again,

% A)

\markup{
        \fill-line { \null \center-column {  wwwwwwwwwwwwww }  \null }
}


% B)

\markup{
         \fill-line {  \bracket \center-column {  wwwwwwwwwwwwww }  }
}


This would be useful for creating centered columns as header titles.
Thanks!

-- 
Francisco Vila. Badajoz (Spain)
www.paconet.org , www.csmbadajoz.com