Gravis spent the next hundred years trying to understand Inferno. As soon as he started, he noticed the difference. Inferno was fire, but it also wasn't fire. Even more surprising was that Gravis' understanding of mid-tier fire gave him nearly no help in understanding high-tier fire. It was like they were made of entirely different things.
From the outside, they looked nearly identical, but there were completely different Laws and concepts inside. In actuality, Gravis' understanding of Punishment Lightning helped him more in understanding Inferno than the normal fire element, which was nearly unbelievable to him.
This was probably also one of the reasons why so few beasts had managed to comprehend the level three equivalents of their elements. The only thing helping them was their affinity, but every other Fire Law was basically useless when looking at this Law.