Technically, we've got two endings—there's the last chapter, and then there's also the epilogue. Why are there two endings? Ah, dear Shmooper, let us lead the way…
Think of the first ending as the chronological ending to the book. All the events in the book lead up to this final moment between mother and daughter, when Young Ju is about to leave for college. What Uhmma gives her is this memory of Apa and Young Ju jumping into the ocean—in other words, a memory of a happy family, of a "family of dreamers." It's a feel-good moment that even manages to bring Apa back into the fold without really bringing him back. (Because really, who would want him back?)
This last chapter does exactly what a typical ending does—it tells us what happens to everyone in the novel.
The epilogue on the other hand, is all about giving us a timeless feeling. We have no idea when the chapter occurs; we just know it's some time in the future and Young Ju and Uhmma are walking on the beach.
Which is kind of strange because the whole chapter is about the importance of Uhmma's personal history: "I study these lines of history [on Uhmma's hand] and wish to erase them… She stares for a moment at her callused skin and then says firmly, These are my hands, Young Ju" (18-19).
It's kind of like An Na is making the whole idea of valuing your history so important that the lesson has to feel timeless and universal. Which is why this chapter has to end the book—it's like the moral of the entire story.