Aligning child's axis to parent's axis - world space: Why does my Quaternion.Angle never return values close to zero when they visually are?

I am aligning an object's +z axes - which points downwards - to the parent's -y axis which points downwards. That is, I want the child object to face down all the time, in respect to parent's object down.

It works well by using the following code:

 // Obtain local forward vector relative to world
 Vector3 localForward = this.child.transform.forward;
 Debug.DrawRay(this.child.transform.position, localForward, Color.blue); // draw axes local space
 Debug.DrawRay(this.child.transform.position, -this.parent.transform.up, Color.green); // draw down axes parent space

 // Building the world (not-local) rotation from child's local z-axis to parent's y-axis
 Quaternion verticalDown = Quaternion.FromToRotation(localForward, -this.parent.transform.up) * this.child.transform.rotation;
 // apply it
 this.child.transform.rotation = Quaternion.Slerp(this.child.transform.rotation, vertical, Time.deltaTime * 1.0f);
    

It works well. However, in the default pose (child object's forward is down and parent's down is down) the following never equates to zero, but 89-91.

 Quaternion.Angle(this.child.transform.rotation, verticalDown);

Why is this so? I'd expect the value would be zero since both child's axes forward is facing down and parent's down is down in the beginning.