how do Slur #'positions work?

[Graham Percival]

Tracking down a potential bug; in this example, both slurs have the same 
height.  Shouldn't the second slur be much higher?  Could the vertical 
spacing code be moving the second slur down?

Cheers,
- Graham


\version "2.11.24"
\relative c'' {
        \slurUp
        \override Slur #'positions = #'(5 . 5)
        c8.( b16) c8. b16
        \override Slur #'positions = #'(30 . 30)
        c8.( b16) c8. b16
}