Our cars have an active engine management system, vs, for instance a '60s car with a carburetor that only has an idle screw on it. Between the Mass Air Flow (MAF) sensors in the intake, and the O2 sensors in the exhaust, the engine management system adjusts the air/fuel mixture to compensate for in-the-moment conditions.
There are also various special load-based conditions, like your A/C, requirements for onboard electronics, and even cold-start and warm-up automatic choke, all of which could affect idle.
Kirk B.
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