It probably depends on how hte throttle control. Most cars after around '01 rpobably have electronic throttle control. There will be less variation with those systems. Before that time, the throttle is tyically controlled by vacuum and is more of a analog closed loop feedback system. It will vary as the load changes before stabilizing.

Ther are other factors. In neutral, a higher idle speed is needed, especially on a I4 because there is only 1 power stroke for every 720 degrees of crank rotation which makes for a relatively unsmooth motion at very low RPM's.