JT Wright wrote:

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

I think I have found the answer:
The altimeter setting value does not take into account the temperature. The
temperature profile of a "standard" atmosphere is assumed. The SLP takes into
affect the actual temperature reading (12 hour mean temperature average). So it
looks like the difference between these two readings can be greater at higher
station elevations. Still, I'm curious about the details of how this is calculated.

http://www.crh.noaa.gov/den/awebshtm...pressure.shtml
http://www.faa.gov/atpubs/SWO/chapter_14Sect1.htm#14-2.%20%20%20ALTIMETER%20SETTING%20(ALTSG)