User blog comment:B214/Question and Answers 6/@comment-29362784-20161221212439/@comment-1359322-20161222024058

They never said all Ultimate-class are Satan-class though. Saying Ultimate-class rivals Satan-class is going to cause misconception to others. Even Kiba said this:

"“Rankings 20 and below are said to have a power on a totally different level, and if you are in Top-10, you would even be called a Hero. Among them, ranks 5 and above are said to be unmovable. They have been standing in the top positions where they haven't moved their ranks for a long time. Especially the 3rd ranked Bedeze Abaddon, the 2nd ranked Roygun Belphegor, and the 1st ranked Diehauser Belial, they are Ultimate-class devils among Ultimate-class devils who have the power equal to the current Maou. Though, all three of them are said to not take any actions unless a large scale War were to happen. They have been researched due to the game’s traits, and they are spoken well of by others. They are called the ‘results’ born due to many matches.”"

So not all Ultimate-class are equal in strength.