One Came Home is what Georgie's Ma cries when she sees Georgie at Grandfather Bolte's funeral: "One came home! One came home! Georgie, you came home!" (20.20). Georgie's return is pretty significant for Ma because the other two members of her family who left (Agatha and Georgie's Pa) didn't come home. So it's a Big Deal that Georgie does.
It's also interesting that the book is about Georgie's journey, but the title emphasizes her return. Titles tell us to pay attention, and this one tells us something key about Georgie: She's going to come home. And for good, too, since she's going to stay and run the store. She loves Placid and wants to stay there.