Why are the airport pressure values for "altitude" (inHg) and "sea level" (mb) slightly different on the NOAA web sites?
The difference has nothing to do with unit conversion (which is correctly done in your question). The difference comes from temperature: the aircraft altimeter assumes a standard temperature lapse rate of 1.98°C per 1000 ft altitude when converting a pressure to an altitude reading. The reference pressure for the altimeter (QNH) is defined such that the aircraft altimeter will show the correct airport elevation when it sits on the ground.
The sea level pressure however takes the actual temperature into account. If this deviates from the expected value in the standard atmosphere, you will see a different pressure value here. The higher the elevation, the more pronounced will these differences typically be. That's probably why you noticed these differences at Denver. In your example, the temperature was 54°F, which is about 12°C, but the expected temperature at Denver is
$$ \vartheta = 15^\circ \text{C} - 1.98^\circ \text{C} \times \frac{5434 \, \text{ft}}{1000 \, \text{ft}} \approx 4.2^\circ \text{C} $$
Here is the full explanation from NOAA:

ALTIMETER SETTING: This is the pressure reading most commonly heard in
radio and television broadcasts. It is not the true barometric
pressure at a station. Instead it is the pressure &quot;reduced&quot; to mean
sea level using the temperature profile of the &quot;standard&quot; atmosphere,
which is representative of average conditions over the United States
at 40 degrees north latitude. The altimeter setting is the pressure
value to which an aircraft altimeter scale is set so that it will
indicate the altitude (above mean sea level) of the aircraft on the
ground at the location for which the pressure value was determined.
The altimeter setting is an attempt to remove elevation effects from
pressure readings using &quot;standard&quot; conditions.
MEAN SEA LEVEL PRESSURE: This is the pressure reading most commonly
used by meteorologists to track weather systems at the surface. Like
altimeter setting, it is a &quot;reduced&quot; pressure which uses observed
conditions rather than &quot;standard&quot; conditions to remove the effects of
elevation from pressure readings. This reduction estimates the
pressure that would exist at sea level at a point directly below the
station using a temperature profile based on temperatures that
actually exist at the station. In practice the temperature used in the
reduction is a mean temperature for the preceding twelve hours. Mean
sea level pressure should be used with caution at high elevations as
temperatures can have a very profound effect on the reduced pressures,
sometimes giving rise to fictitious pressure patterns and anomalous
mean sea level pressure values.

(NOAA - Pressure Definitions)