That is a question nobody has been able to answer to me.
Actually it can not be the pressure at the airport. Take a look at this METAR from Denver International.
KDEN 142153Z 02013G16KT 10SM SCT090 BKN140 BKN220 34/M04 A2999 RMK AO2 SLP061 T03391044
It says the altimeter setting is 29.99 inches of mercury. We all know that altimeter setting ( or QNH) is the station pressure corrected to sea level. Station pressure is called QFE and in the case of Denver (5200 feet MSL) it would be somewhere around 24 inches of mercury or 870 milibars. It is impossible to have 29.99 at 5200 feet ( field elevation).
If you continue reading, in the remarks section it says SLP (Sea level Pressure) 061, which means 1006.1 milibars or hectopascals. And that, of course, it is not the conversion from 29.99 inches.
So what does SLP mean in US Metars?
AC00-45 Aviation Weather Services say "some stations include the sea-level-pressure, which is different from altimeter".