IFR requirement for altimeter question (i.e. +/- 75 feet) What if the ATIS/AWOS/ASOS results in a very low or very high altimeter setting (i.e. barometric pressure)? Surely this setting will result in an altimeter setting way outside the envelope of the +/- 75 feet related to field elevation? Since each inch of Hg results in 1,000' change...what am I missing w.r.t. the altimeter check?
Barometric pressure isn't a constant. Just because you're at sea level, doesn't mean the air pressure is going to be exactly 29.9212 inHg. That's just the average sea-level pressure. So, for instance, if the sea level pressure is currently 30.9 inHg, then all altimeters in the region are going to read 1000 feet below the actual altitude of the airplane.
Those fluctuations are the point of the altimeter corrections provided by AWOS/ASOS/ATIS. By entering the actual sea-level atmospheric pressure into the altimeter, you can correct for those fluctuations and get a more accurate reading of your current altitude.