It was a mistranslation. I remember when I bought the PC version of FF7, the name by default was still 'Aeris' , but if you look in the box in the back there are images of fight scenes. One of which calls her by her real name, 'Aerith'. Therefore you can see they were conflicted in what to call her. Yet I still don't know why they would not change the name in the PC version. Maybe they just thought people got used to 'Aeris' so they kept it that way.