My theory is quite simple. In Romance Dawn Luffy have eaten Devil Fruit and at that point we were told, that is a Gomu Gomu no mi. For more than 1000 chapters that was the canon. Currently there are reasons to believe that is not truth, but I think that it might be a red herring. Oda does that sometimes. I think that at that point of the story, Luffy was already DF user. We know that he was not able to swim. I think that for some reasons, he is able to have two DFs, but one of them is dormant or semi-dormant. That DF fruit would be Sun DF or Melt DF. Sun works better in the story because of the Sun connection and Black Beard Devil fruit.