The movie scriptwriters are to blame for making this unclear; but the term "Narnia" does not equally describe ALL of the Narnian world. Only Narnia is Narnia. Neither Archenland, nor Calormen, nor the territory of giants, is a Narnian kingdom; there is only one Narnian kingdom. That's why, in my own fanfiction, I use the phrase "the Narnian world" to distinguish when I'm talking about the whole "flat planet" that Aslan created.