Initially, Nickelodeon picked up Korra as a 12-episode miniseries. Their idea was to do 12-episode arcs that were more standalone than the original Avatar series. Mike and I were cool with this idea, as we had originally wanted the seasons to be 12 episodes long instead of 20, and creatively we liked the idea of doing tighter story arcs.
The original plan was that if Nickelodeon wanted more episodes, they would order them 12 episodes at a time. But while we were in production on Book 1, Nickelodeon decided to change their season lengths from 20 episodes (like Avatar had) to the more standardized international length of 26 episodes. They liked how Book 1 was coming out and decided to round out the order to fit their new 26-episode mold, and we got a 14-episode pick-up, which became Book 2.
Then, the network wanted even more Korra, so they picked up another order of 26 episodes. Mike and I wanted to stick with the shorter “books,” so creatively we are splitting the second order of 26 into Books 3 & 4, 13 episodes each.
What makes this even more confusing is that the network considers each block of 26 episodes a “season,” which is another reason we try to stick to calling these Korra arcs “books.” So for the network’s purposes, Books 1 & 2 are Season 1, and Books 3 & 4 are Season 2.