Altimeter setting != Sea Level Pressure - Why?

I have a question regarding the relationship between an altimeter
setting and sea level pressure.

Here is the current METAR for Abilene Regional Airport, Texas:

KABI 280152Z 17014KT 10SM CLR 19/16 A2987 RMK AO2 SLP098
T01940161

As you can see, the altimeter setting is 29.87" and the Sea Level
Pressure is 1009.8 millibars (hectoPascals)

But the altimeter setting 29.87" is 1011.6 milibars, or 1.8
millibars higher than the sea level pressure.

Why is there a difference between these two values? What
accounts for the 1.8 millibars?

I've looked at similar METARs, and some with a temperature
greater than standard temp have a lower difference between alt
settings and SLP, so this apparently does not account for the
difference.