Why is the book Extras so different than the preceding books written by Scott Westerfeld? (Uglies Series)?

Like the first three were about Tally, and now it's about Aya. When I took Specials out of the library, it said The Finale to the Groundbreaking Series, but is Extras not the final book? OR is it kind of like what Twilight did with Bree Tanner, and all that?