That's actually a good point you brought up here. If we look at what we were told, Enryu's feat seems to be (far) greater than Phantaminum's. But this perspective may change once we get more information about Phantaminum's assualt on Zahard's palace. While the incident on the 43rd Floor seems pretty "straightforward", the aussault on Zahard's palace is shrouded in mysteries. We don't know whom Phantaminum fought, what kind of op-ness he displayed and why he didn't fight Zahard when he met him. In that regard, the Ranking Administration Office (RAO) seems to know much more than we do for now, thus ranking Phantaminum above Enryu. This is further proven by this fact:I don't think the 10 great leaders are as powerful as Enryu to come near killing the admins. Enryu is an odd case in which i am kind a surprised almost always that the tower which doesn't know what an exis is and keeps Phanta no 1 while Enryu no 2. Phanta at best killed say 100 high ranker. Big deal Enryu killed a whole frigging guardian.
It would have been sensible if 2-3 family head were killed zahard ran for his life etc for Phanta to be no 1
ToG Wiki:
So, they indeed know much more about the incident than SIU has told us so far. If someone casually says to Urek's face that he won't stand a chance against Phantaminum, then they were surely impressed by Phantaminum's power.There is a fun story from the rank giving process: there was a time when Urek Mazino, who had a complaint about his rank being lower than Phantaminum's, broke into the Ranker Administration Office, captured Aisand and threatened him to raise his rank. To Urek, who asked "Have you ever seen me fight Phantaminum?" Aisand answered briefly and without hesitation "You would be the loser". Urek Mazino, shocked, could do nothing but go home bitterly.
By the way, SIU also implied that it was a very tough call within the RAO to put Phantaminum on the first place, which means that the RAO doesn't see much of a big difference in power between Enryu and Phantaminum.
