Her sister looks quite a bit older than her, and her parents must be older still than that. Perhaps it has to do with them, hand me downs aren't exactly rare. Or it's just something to do with the author. It wouldn't be weird for that to be their birth year, that'd put them at about 29 years old or so right now if so. Incidentally, that one teacher is 29 too, perhaps that was also a self reference by the author. Not that it really matters in any event.Why is a loli wearing a shirt that says 1991 on it