Its nothing to do with psi. it's when talking about horsepower and torque etc there are two options on the car I'm looking at, one says 163 PS and the other 184PS
Click http://en.wikipedia.org/wiki/Horsepower for a definitive answer.
Seems to be a way of measuring power transmitted to the road wheels as opposed to power actually produced at the engine.
Happened to be glancing at the Golf R32 on the Volkswagen website and I quote:
"The maximum power output figures are quoted in PS (or Pferdest�rken), which is the metric equivalent of horsepower. To convert from metric to imperial horsepower, divide the PS figure by 1.0139."