The standard barometric pressure of 29.92 inHg or 101.3 Kpa is used on all aircraft above 18000' (normally) to ensure everyone's altimeter displays the same altitude. So when you are landing, the baro pressure at the airport is not usually ever 29.92 inHg. When you set the DH it is usually based on the radar alitmeter which does not use the baro pressure, in order to see changes in terrain more precisely. I hope this helps?  ;)