Why does the player jump is not so high if he touch 2 objects at the same time as if it touch only 1 object?

  void OnCollisionStay(Collision collisionInfo)
     {
       
 
         if (collisionInfo.gameObject.tag == "jumpable_thing")
         {
             CanJump = true;
       
         }
     }
 
  void OnCollisionExit(Collision collisionInfo)
     {
         CanJump = false;
         jump = false;
     }
 
 void FixedUpdate ()
     {
    
         float moveHorizontal = Input.GetAxis("Horizontal");
         float moveVertical = Input.GetAxis("Vertical");
 
         
         Vector3 movement = new Vector3(moveHorizontal, 0.0f, moveVertical);
 
         rigidbody.AddForce(movement * speed);
 
 
         if (jump == true)
         {
             rigidbody.AddForce(Vector3.up * jumpSpeed);
             CanJump = false;
             jump = false;
         }
 
     }