Setting the midiInstrument to "woodblock" ruins the woodblock sound?

Hi,

Thinking I was doing the "right thing" I added `\set
Staff.midiInstrument = "woodblock"`
to a "metronome" snippet. I was puzzled when wbh and wbl sounded identical. Lo
and behold, when I removed it, the two sounded quite different.

This seems very counterintuitive and confusing to me. Why would telling it the
instrument o use break it?

%%%% Fails %%%%
metronome = {
  \global
  \set Staff.midiInstrument = "woodblock"
  \drummode {
    \repeat volta 66 {
      wbh4 wbl4 wbl4 wbl4
    }
  }
}

%%%% Works %%%%
metronome = {
  \global
  \drummode {
    \repeat volta 66 {
      wbh4 wbl4 wbl4 wbl4
    }
  }
}