Differences in altimeter to sea level pressure at higher elevations

[DaveJ83]

I watched metpy monday 242 and also coded it myself. At the Denver airport the calculated slp and the slp in the metar were different by 5 or 6 mb. Airports at lower elevations are more in line with each other. He said he was doing some investigation on it. Is there an update or a way to bring the calculated more in line?