unity – When the player is standing on a terrain he is changing position he is moving a bit but moving. Why?

I did a small test.

When the player is standing on a terrain he is changing position a bit but changing I mean you can see the player is moving a bit but moving. He is not standing on place.

When the player is standing on a Plane he is not moving at all.

I will show some screenshots.

First the player inspector settings :
The player is a character I bought some years ago from the store it’s not my character.

The player have this components by default :

Animator , Rigidbody , Capsule Collider , Third Person User Control , Third Person Character

Player Settings

The player default state in the animator controller is a blend tree that play Humanoididle animation.

The next screenshot is the settings of the terrain :

Terrain Settings

And the Plane is a regular plane I added for testing just did GameObject > 3D Object > Plane

And on the Plane the player is not moving he is standing still while playing the idle animation.

But on the terrain the player is moving like sliding. It’s small movement but after some minutes the player is in another place on the terrain, the player is moving.

I can’t figure out why on the terrain he is moving a bit and on the plane he is standing still.
I want the player to be on the terrain but not moving when the idle animation play.