Gravis knew how his body, lightning, and Spirit worked the best. Therefore, he could also deduce what had happened. The growth process of his Spirit and body went through something that Gravis called hard breakthroughs, while his lightning grew via soft breakthroughs.
He had come up with these terms after learning smithing with his father and remembering how he had grown in the lower world. When his Spirit and body received some tempering, their power didn't increase immediately. It always required them to amass enough resources to break through into the next level in one motion. This was what Gravis dubbed as a hard breakthrough.
Meanwhile, ever since Gravis got his lightning, it never went through these kinds of hard breakthroughs. It just grew autonomously. There were no boundaries, borders, or direct Realms that he could assign to his lightning. It simply became more powerful. This was what Gravis dubbed as a soft breakthrough.