The last couple of flights I've had I noticed the altimeter did not match the field elevation when inputting the current baro pressure from ATIS. In fact it was off by 150-200ft. I'm embarrassed to admit I can't figure out why. I was trying to equate it to temperature, but it would seem the pressure at a given level, is the pressure regardlesss of temp. The field is basically at sea level (10ft) yet the altimeter typically indicates 200ft below this. It makes me wonder if I should be accounting for this "error" or assume the altimeter setting is correct and fly at assigned altitudes regardless. I did some googling to no avail so hopefully this question isn't too dumb!