Understanding spacers err.. spacing

[James Lowe]

Hello,

I had been trying to get a fermata to align centrally over the last bar which 
is just a full rest (yes I know it makes no sense musically but anyway..) I had 
been struggling.

So I came up with the idea of using spacer rests and you cannot

do

R1^\fermata

But even with spacer rests I couldn't get it to work without manual shifting. 
However if I use real notes instead of spacers it seems to work so I wondered 
if anyone could explain why the fermata is not aligned when using spacers only 
as it is when using real notes in the example below because I was expecting a 
spacer in this context to act like a note at least in spacing within a measure.

I hope that makes sense

--

\version "2.13.9"

Music = \relative c'' {
  \time 9/8
  << { R4.*3 } \\ { c8 c c c c^\fermata c c c c } >> % This works
  << { R4.*3 } \\ { s8 s s s s^\fermata s s s s } >> % This is off-centre
  << { R4.*3 } \\ { s8 c s c s^\fermata c s c s } >> % This works
}

\score {
    \Music 
}

--

James