I think it's due to the confusing terminology. What Americans refer to as a "season" is called a "series" by the Brits (I don't know how Australia falls on this, but here in Canada, we go by the American convention.)
So Synchronicity was the final epsiode of the third season (or series), but probably not the final episode in the show's overall life. A fourth season has apparently been comissioned and will apparently adapt The Torment of Others (the 4th book in the book series) as part of it.