How is a high altitude natural gas burner different?
The basic chemistry of efficient natural gas burning is about a 10:1 air to fuel ratio.  The air at high altitudes has less oxygen per volume (the air is less dense)

There are a number of ways to compensate.  


Derate the burner by decreasing the fuel orifice size.  This returns the burner to the correct air-fuel ratio, but produces less heat.  This method is going out of favor in higher efficiency appliances.
Add air to the natural gas.  You are in effect derating the gas content to get the proper air-fuel ratio.  This is done at the utility, your appliance vendor will know what percentage is added (if any) in your area.