Change in RPM in park

[DipnX5]

Ok so here is my interesting experience.

I was sitting in target parking lot waiting for a friend. The truck was on with the front wheels slightly against the curb. It was in park and bored, as I was turned the wheel for no reason at all, I watched my RPM gage decrease and increase as I turned the wheel from all the way to the left to the right.

This is the first time I have noticed the RPM gage moving like that in park without having my foot on the accelerator.

Any ideas? Is this a clue I am up for some rough waters at the stealer?

Thanks in advance your your thoughts,

Austin