First of all, the ending sets up the beginning of the next book. Carmen explains that the Pants will be taken out next summer and start all over again, and with that, we know there's at least one more book to come in the Sisterhood's saga (though actually there are a few more).
As the story ends, the girls have finally reunited, but there is no denying that their friendship doesn't feel quite the same. They each changed over the summer, and a lot remains unsaid between them about their experiences—but it's okay. With the Pants, which "absorbed the summer" (Epilogue.8), their friendship isn't threatened—the Pants hold all of their individual threads together in one place.
The ending is distinct because of one word: tone. Carmen's voice has changed. Go back and skim the Prologue and you'll see what we mean—at the book's end, she is wiser, more contemplative, and definitely more mature. The experiences the girls have apart from each other force them to grow up a little and change, and when they come back together, they aren't as innocent as they used to be.