Why Child's Local z position is changing when I change z of my camera.

I have a scene where I have my background sprite positioned at 10 local unit z under camera. I have a perspective camera. My background is child of my camera. I am changing z of my camera but it's automatically changing z of background. Like when I change my z to -20 camera child background object's z also change to 20 How can I avoid it ?

My Camera Movement / Following script

 using UnityEngine;
 public class CameraController : MonoBehaviour
 {
     Vector3 newPos;
     public Vector3 camOffSet = Vector2.zero;
     public float speed = 2;
     Vector3 TargetPos
     {
         get
         {
             return CarController.Instance.transform.position;
         }
     }
 
     void FixedUpdate()
     {
         if (CarController.Instance != null)
         {
             newPos = TargetPos + camOffSet;
             //if (!CarController.Instance.IsGrounded && CarController.Instance.CanZoomOut)
             //{
             //    float zz = Mathf.Clamp(-10 - newPos.y, -20, -10);
             //    newPos.z = Mathf.Lerp(transform.position.z, Mathf.Clamp(-10 - newPos.y, -20, -10), speed * Time.deltaTime);
             //    //newPos.z = zz;
             //    newPos.x -= zz * 0.2f;
             //}
             //else
             {
                 newPos.z = Mathf.Lerp(transform.position.z, -10, speed * Time.deltaTime);
             }
 
             transform.position = newPos;// Vector3.MoveTowards(transform.position, newPos, speed * Time.deltaTime);
         }
     }
 }
 

My Bg Move/follow script

 public float speed;
     Vector3 targetPos = Vector2.zero;
     
     void Update()
     {
         if (CarController.Instance != null)
         {
             transform.position = Vector2.MoveTowards(transform.position, targetPos, speed * Time.deltaTime);
         }
     }