Board Thread:New on Sword Art Online Wiki/@comment-115.133.52.60-20170221224427/@comment-6697950-20171008102212

Simply put, SAO was technically supposed to be a game. Nobody knew what Kayaba was planning, so nobody knew that he was going to make himself the boss. But, as SAO was marketed as a regular game, it had to have all the bosses set up in advance, including a boss for the 100th floor. That's the monster that came up in Ordinal Scale.