So, Sasshi always wanted the world to be exciting. However, after preventing his grandfather's death by creating a whole new world where he's not dead and moving everyone around and such, he forgets the whole thing and becomes more mature, losing his innocence and his affinity for all of his childhood things. In the creation of the new world, that part of his consciousness is shunted into the creation of Haruhi as a small child with a proper history in the nearby (about 10 miles away) city of Nishinomiya, which is in the same greater metropolitan area anyway. Since the world was created from whole cloth, it's relatively easy for Haruhi to change.