Why Camera.ScreenPointToRay is not accurate in 3d world?

Here is the video

I'm trying to hover the player over the cursor, but it doesn't work exactly, here's the code:

var playerPlane = new Plane(Vector3.up, transform.position);

// Generate a ray from the cursor position
var ray = Camera.main.ScreenPointToRay(Input.mousePosition);

// Determine the point where the cursor ray intersects the plane.
// This will be the point that the object must look towards to be looking at the mouse.
// Raycasting to a Plane object only gives us a distance, so we'll have to take the distance,
//   then find the point along that ray that meets that distance.  This will be the point
//   to look at.
float hitdist = 0.0f;
// If the ray is parallel to the plane, Raycast will return false.
if (playerPlane.Raycast(ray, out hitdist)) {
    // Get the point along the ray that hits the calculated distance.
    var targetPoint = ray.GetPoint(hitdist);

    // Determine the target rotation.  This is the rotation if the transform looks at the target point.
    var targetRotation = Quaternion.LookRotation(targetPoint - transform.position);

    // Smoothly rotate towards the target point.
    transform.rotation = Quaternion.Slerp(transform.rotation, targetRotation, speed * Time.deltaTime);
}

Here is the player hierarchy The player and his scripts:(further child objects)

• mesh
• the same mesh that changes position depending on the rotation of its parent