I had assumed during a WAAS approach when the GPS is in LPV mode, both lateral guidance and vertical guidance are generated from GPS/WAAS signals and the pitot-static system should have no effect on its vertical guidance. This is in contrast with LNAV/VNAV approach, where the vertical guidance comes from the barometric system, the same source as altimeter. 

However, when I read [RNAV (GPS) Z RWY 30 @ KHAF][1], I will get a different DA if I use a different altimeter setting. So, why does altimeter settings matter to the DA in an LPV approach?  


  [1]: http://flightaware.com/resources/airport/KHAF/IAP/RNAV+(GPS)+Z+RWY+30