There's a really good explanation of the seasons a page or so back, but the gist is Nickelodeon ordered 12 episodes (book 1) originally and then ordered 14 more episodes (book 2) for a 26 episode season 1. Book 3 & 4 will be season 2. For Nickelodeon's tv purposes, a season is 26 episodes long, but the creators want to stick to 13 episode story arcs; hence, multiple book arcs per season.
I'm so glad I'm not the only one who thought that I was like "Oh hey another all powerful wind chime."