stem length in staff space

[Klaus Rettinghaus]

Hi all, 

Internals Reference says stem.length measures in staff spaces. But actually it 
goes by half staff spaces. 
So either the documentation is wrong or this is a bug. 

\version "2.18.2" 

\relative {
  \override Stem.length = 0 g' 
  \override Stem.length = 2 g 
  \override Stem.length = 4 g 
  \override Stem.length = 6 g 
  \override Stem.length = 8 g 
  \override Stem.length = 10 g 
}

Regards 
Klaus