There is a potentiometer in the servo that tells the servos processor that it is in the right position. Since the processor uses the pot when the servo is in motion or when the servo is holding a force against external force then it has to do a couple things.

sit at 0 speed but hold with force
be at a velocity represented by a series of positions with a force
Correct any mistakes that it has made or not been able to respond to

it corrects itself by bumping the current one way or the other and watching the pot response. That amount of current it takes to move the wheels on the track is more than it is in the air. The track damps the mechanical response. Without the track dampening the corrections it gets over corrected. The pot then over corrects it back in the opposite direction.

you will see settings in the PGS that control the corrections in the initial, mid, and end portion of the motion. If you want a responsive car you tune these parameters to the point of oscillation on the ground and then back off 10%.

This means it will oscillate when its not on the ground.