why doesn't Y-offset work for Hairpins?

[Marcus Macauley]

The documentation page for Hairpin --
http://lilypond.org/doc/v2.9/Documentation/user/lilypond-internals/Hairpin.html
-- lists Y-offset as one of the standard settings. It also says that  
Hairpin supports grob-interface, and the grob-interface page --
http://lilypond.org/doc/v2.9/Documentation/user/lilypond-internals/grob_002dinterface.html
-- also lists Y-offset as one of the "User settable properties".

So I would assume that putting

        \once \override Score.Hairpin #'Y-offset = #6

before a hairpin object would shift it up or down, in this case by 6 staff  
spaces.

But in fact, the following example moves the hairpin not at all:

        \version "2.9.17"
        \layout { ragged-right = ##t }
        {
                \once \override Score.Hairpin #'Y-offset = #6
                \relative c' {c1\> c1\!}
        }

Am I missing something, or is this a bug?

BTW, X-offset works just fine. And extra-offset works too, but I don't  
think that's what I want (because it doesn't create any extra space).

Marcus