User blog comment:Miskos3/Fairy Tail 514: Dragon Seed, Review/@comment-9595671-20161215054921

I don't think Erza's necessarily a Dragon, if she was in Irene's womb at the time she had to have developed as a Human. When Irene began to transform, it's likely those dragon traits enhanced how she developed.

So maybe Erza's a human who's body composition has traits of a dragon. That enhanced composition manifested as great magical potential and physical strength as she began to develop it further and further.