Why is it elegant when moving models

This is how I move the model

void OnMouseDown()
{
distance = Camera.main.ScreenToWorldPoint(new Vector3(Input.mousePosition.x, Input.mousePosition.y,Camera.main.WorldToScreenPoint(transform.position).z)) - transform.position;
}

void OnMouseDrag()
{
Vector3 distance_to_screen = Camera.main.WorldToScreenPoint(transform.position);
Vector3 pos_move = Camera.main.ScreenToWorldPoint(new Vector3(Input.mousePosition.x, Input.mousePosition.y, distance_to_screen.z ));
transform.position = new Vector3( pos_move.x - distance.x , transform.position.y, pos_move.z -distance.z );

}


In the scenario the moving model will float and why is that