The Snecma-Larzac 04-C6 idling RPM ranges from approximately 53% on the ground to 85% at high altitude. Why is that?
Ground idle and in-fight idle are different. In-flight idle would move aircraft on the ground and would require a bit of brakes to keep the aircraft stationary. Additionally, landing roll would be longer. Therefore, ground idle activates on most jet aircrafts after touchdown. This activation is usually occurs after a small delay, taking into account that pilot may be doing a touch and go.
Of course there is also density effect on both ground and in-flight idle rpms. Both increase with altitude to provide some engine stall margin.