Sometimes, when IDLE voltage is LOW, ECU rises RPM to increase current generation.
Let's say, if you keep the engine idling for a long time (1-2 minutes it's long) with lights, hazard lights, a/c etc on, engine will increase RPM from ~650 to ~1000.
ECU it's going to keep that minimum idle (1000rpm) for some time even after you start driving.
(that applies to starjet ecu, it's on the specs)
Also engines' minimum RPM is ~1000 with the car rolling (without gear) until it's almost stopped. Then minimum rpm goes to 650 again.