User blog comment:IamJakuhoRaikoben/Chapter 515: I Am You... You Are Me, Review./@comment-117.223.154.228-20161220220753/@comment-7418318-20161221163927

There are some differences. First the DS have been only around for 7 years when Acnologia first turned up, another 7 at his second showing and another year for his final appearance, while Irene was flying around for 400 years.

But what's more important is that Acnologia very specifically didn't include Irene into his head-count. That he didn't hunt the other DS at every opportunity can just be put down to something as trivial as mood if nothing else, but he specifically counted 7 "Dragons", so either he overlooked (oversmelled?) Irene or one of the other DS.

It'd be funny though if he got lonely after killing off all female Dragons and thought to take the last one as his mate and that's why he deliberately didn't include her as one of the Dragons he has to kill desite knowing of her. ^^