PS2727 wrote:

Altimeter settings are corrected for elevation of the station. Imagine an
airport which is at 1000 ft. in elevation. On a standard day the altimeter
setting on ATIS will say 29.92, but the actual air pressure is closer to 28.92
(using 1in/hg per 1000 ft). Yet when set to 29.92 your altimeter will read
field elevation of 1000 ft when you land.

That's certainly true, but it's not related to my question. Both the altimeter
setting and the SLP are corrected for the elevation of the station. My question
relates to why sometimes (but not always) there is a difference between these
figures.

For example, the latest METAR from KABI shows that the difference between the
altimter setting and the sea level pressure (SLP) has increased by 0.1 mb in the
past few hours.


KABI 280452Z 17017KT 10SM FEW032 18/16 A2989 RMK AO2 SLP104 T01830156
Altimeter 29.89 inches Hg or1012.3 mb, Sea-level pressu 1010.4 mb:
Difference of 1.9 mb


earlier METAR in previous post:
KABI 280152Z 17014KT 10SM CLR 19/16 A2987 RMK AO2 SLP098 T01940161
29.87" (1011.6 milibars) and the Sea Level Pressure was 1009.8
Difference of 1.8 mb