Setting staff size for individual scores

[Patrick Karl]

Reading the documentation in Notation:4.2.2:  Setting the staff size, one thinks that one can set the staff size for all scores in an input file by using the set-global-staff-size function, or one can set the staff size for each individual score in an input file by using the layout-set-staff-size function.   Then comes the "Known issues and warnings" subsection:  layout-set-staff-size does not change the distance between the staff lines.

I find that a little confusing.  How can layout-set-staff-size change the staff size without causing the distance between the staff lines to change?  It appears that all layout-set-staff-size does is choose the font(s) that would be needed for the specified staff size without changing the staff size at all.  One can produce some really ugly engraving with, e.g., the following snippet:

music = \relative c' {
        a b c d e f g a
}
\score {
        \new Staff {  \music }
        \layout { #(layout-set-staff-size 60) }
}

On the other hand, the following snippet produces beautiful engraving:

#(set-global-staff-size 60)
music = \relative c' {
        a b c d e f g a
}
\score {
        \new Staff {  \music }
        \layout {}
}

So, I guess my question is:  if set-global-staff-size can change the distance between staff lines to match the fonts chosen, why can't layout-set-staff-size do the same?  Or, alternatively, why does layout-set-staff-size even exist if it can't do that?