The way the believers view a god determines what is compatible with the god. Zernon is seen as a strict god with rules and regulations. He likes law and meting out justice. He protects the mortals by keeping order in the mortal realm. He is the god of order and justice. His believers don't see him as a sunny god that brings life and light. He doesn't invoke happiness in them as the sun god does. He is more like a strict father that is always frowning at misdeeds and punishing rule breakers so the domain of the sun is not compatible with his divine image.
The god of the sun on the other hand is ancient. He is very powerful and very wise. He brings light and life. He and the domain of order is not a perfect match but they can see him using his incredible power to maintain order in the plane. So the domain of order is compatible with him. He is fusing with it and will then become a level two Celestial.
