how to calculate rotation

I have a problem that I cannot use the rigidbody. When the ball rotates with angular velocity, you have to calculate routing. I'm stuck here.

 Rigidbody rigid;
     public Vector3 startVelo;
     public Vector3 startAngularVelo;
     public Vector3 startForce;
     // Start is called before the first frame update
     Quaternion preRot;
 Vector3 prePos;
 Quaternion oldRot;

 void Start()
 {        
     rigid = GetComponent<Rigidbody>();
     rigid.maxAngularVelocity = 1000.0f;
     rigid.velocity = startVelo;
     rigid.angularVelocity = startAngularVelo;

     preRot = transform.rotation;
     oldRot = new Quaternion(0f, 0f, 0f, 1f);
 }        

 private void FixedUpdate()
 {        
     Quaternion deltaRotation = transform.rotation * Quaternion.Inverse(preRot);
     preRot = transform.rotation;

     float angle = 0.0f;
     Vector3 axis = Vector3.zero;
     deltaRotation.ToAngleAxis(out angle, out axis);
     angle *= Mathf.Deg2Rad;
     Vector3 angluarVelo = axis * angle * (1.0f / Time.deltaTime);

     Quaternion rotation = Quaternion.AngleAxis(angluarVelo.magnitude * Time.deltaTime, angluarVelo) * oldRot;
     oldRot = rotation;
 }

The value of oldRot and transform.rotation is different. Do you know why?

I am sorry for my poor English. Please help me.