StaffSymbol: behaviour of ledger-line-thickness

[till Rettig]

Hi,
could somebody explain me how ledger-line-thickness behaves? The IR states that 
it should be a pair: 

"ledger-line-thickness (pair of numbers)
    The thickness of ledger lines. It is the sum of 2 numbers: The first is the 
factor for line thickness, and the second for staff space. Both contributions 
are added. "

But I cannot get the staff space bigger (the second number), instead the first 
number influences the thickness of the ledger line a bit, the second quite 
much, that is it becomes so heavy that the spaces almost disappear.
Please compare the example:

\score{
\new Staff \with {
  \override StaffSymbol #' ledger-line-thickness  = #' ( 1 . .1 )
  }{
  d d d d
} }


\score{
\new Staff \with {
  \override StaffSymbol #' ledger-line-thickness = #' ( .1 . 1 )
  }{
  d d d d
} }


\score{
\new Staff \with {
  \override StaffSymbol #'  ledger-line-thickness = #' ( .1 . .1 )
  }{
  d d d d
} }

\score{
\new Staff \with {
  \override StaffSymbol #'  ledger-line-thickness = #' ( 1 . 1 )
  }{
  d d d d
} }

What am I missing here? How is this supposed to work?
I was on 2.11.34, if it matters.

Thanks
Till
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