Object rotate to target in 2D? Not working properly.

I search the forum and found this: (I'm confused/curious)

http://answers.unity3d.com/questions/650460/rotating-a-2d-sprite-to-face-a-target-on-a-single.html

So I made my code just like this:

 Vector3 vectorToTarget = transform.position - targetObject.transform.position;
 float angle = Mathf.Atan2 (vectorToTarget.y, vectorToTarget.x) * Mathf.Rad2Deg;
 Quaternion q = Quaternion.AngleAxis (angle, Vector3.forward);
 
 transform.rotation = Quaternion.Slerp (transform.rotation, q, Random.Range (0.1f, 0.2f));

The result: my object (enemy) facing 90 degrees (offset) right to the target (player). I want to know what's going on here? Why is this happening?

Instead I made: q * Quaternion.Euler(new Vector3(0f, 0f , 90f))in order just to work properly?

 Vector3 vectorToTarget = transform.position - targetObject.transform.position;
 float angle = Mathf.Atan2 (vectorToTarget.y, vectorToTarget.x) * Mathf.Rad2Deg;
 Quaternion q = Quaternion.AngleAxis (angle, Vector3.forward);
 
 transform.rotation = Quaternion.Slerp (transform.rotation, q  * Quaternion.Euler(new Vector3(0f, 0f , 90f)), Random.Range (0.1f, 0.2f));