User blog comment:B214/Chapter 510: Natsu's Heart/@comment-97.104.204.83-20161114104237/@comment-7418318-20161114113453

Since this all happened in Natsu's head, I wonder if maybe his brain somehow patched the missing images in with people Natsu knew rather than that being the actual appearance of his parents.

Sure Zeref said that's what they looked like, but who's to say that Zeref and Natsu saw the same thing... or that this really is Zeref himself to being with, rather than some subconscious part of Natsu himself. After all that clearly wasn't the real Sting either.