User blog comment:B214/Volume 21 info & spoilers/@comment-27232228-20160410174432/@comment-82.199.213.129-20160410212108

Well, that makes sense, if Issei could just kill Indra with one fire breath, there would be no point in the series, so Ishibumi gave it different levels so that he has to grow, to achieve that power, most likely against Apophis, he temporarily gained access to the highest level.