Cannot get dynamics centered in piano staff

[Giampaolo Orrigo]

I have this score:

\version "2.18.2"

\language "english"

global = {

  \key b \minor

  \time 4/4

}

right = \relative c'' {

  \global

  % Music follows here.

  <<{ a1\> fs2\p r2 s1} \\ {f1 r8 fs->( d b) e->( cs a) \slurUp d->( \change Staff = left \stemUp b8 g) cs^>( a fs) b^>( g e)} >>

  

}

left = \relative c' {

  \global

  % Music follows here.

  g,!?8 d' f b~ b4 g, 

  << {r8 fs'->( d b) e->( cs a) \slurDown d->( \stemDown  b8 g) cs_>( a fs) b_>( g e)} \\ {fs2 r s1}>> 

  

}

\score {

  \new PianoStaff \with {

    instrumentName = "Piano"

  } <<

    \new Staff = "right" \right

    \new Staff = "left" { \clef bass \left }

  >>

  \layout { }

}

For some unknown reason I cannot get the dynamics centered in the middle of the piano staff.

What am I missing?

Giampaolo Orrigo

"Whereof one cannot speak, thereof one must be silent"

(L. Wittgenstein, Tractatus Logico-Philosophicus, Proposition 7)