What happens between Light transfrom rotation and _WorldSpaceLightPos0?

So I'm working on a procedural skybox shader which is basically a customization of the one I found here:

https://github.com/keijiro/UnitySkyboxShaders/blob/master/Assets/Skybox%20Shaders/Horizon%20With%20Sun%20Skybox.shader

This one uses a custom vector for the sun direction, where I pass in the normalized worldspace rotation of my directional light (directionalLight.transform.rotation.eulerAngles.normalized). I'm on forward rendering here.

However if I use the built-in variable _WorldSpaceLightPos0 I get a different result. There seems to be an offset and sometimes axes appear to be flipped.

So what is it that Unity does internally with the rotation of my directional light to calculate the _WorldSpaceLightPos0? And how do I get my light direction vector and the _WorldSpaceLightPos0 to match.

Appreciate any help.