Could be normal or a small sensor or vaccum leak issue. A out of range sensor might not pick up the engine rpm quite right all the time. But not enough to set a code. Intake and engine temp sensors also often set the mix too lean in this situation. It is normal to lean out the engine with throttle closed, but you may have an issue that the IAC is not getting or properly responding to a stall condition. Slow IAC or near failing sensors.