Comment #6 on issue 2462 by [email protected]: unnecessarily wide spacing caused by Lyrics
http://code.google.com/p/lilypond/issues/detail?id=2462
The point is that a "real full line of music" is quite often less than a full line. In other words, what constitutes a full line is a result of stretching something that would naturally be spaced tighter. The measures don't fit line width perfectly; usually some gap is left which is eliminated by stretching the line.
