Calculating angles using Atan2

Hopefully I'm asking this in the correct place. I'm following a Unity tutorial and I'm having trouble reconciling two methods of calculating angles.

In one tutorial, the tutor uses the following line:

Code (CSharp):

1. float angle = 90- Mathf.Atan2(dirLookTarget.z, dirLookTarget.x) * Mathf.Rad2Deg;

He didn't explain why he substracted the calculation from 90, so I sought out a more in depth tutorial on angles specifically. In this tutorial, the tutor uses the line:

Code (CSharp):

1. float angle = Mathf.Atan2(dirLookTarget.z, dirLookTarget.x) * Mathf.Rad2Deg -90;

Why in one case does the tutor subtract the calculated angle from 90 and in the other case subtract 90 from the calculated angle? If someone could help me reconcile both approaches I would be extremely grateful.

 

Sorry I'm no expert on trigonometry but the circle is divided into 4 quadrants each being 90 degrees. That probably has something to do with it. It's difficult to appreciate what the context of each equation is above.
Have a a look at the below page with the interactive diagrams. Good luck!
https://www.mathsisfun.com/algebra/trigonometry.html

 

Why wont u use Vector3.Angle?
or even better dot Product:
https://docs.unity3d.com/Manual/UnderstandingVectorArithmetic.html