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The quote in question:
The Avatar: the Last Airbender universe, tightly packed with vast Asian mythology and likeable character drama, has a number of speech bubbles left to fill between now and the television premiere of Legend of Korra, now scheduled for 2013.
If this is true, the answer as to why could be the fact that Nickelodeon (the network that will air the show) recently has increased the episode count for the once mini-series event (originally it was meant to only be twelve episodes) into a full order of twenty-six episodes (two seasons spanning thirteen episodes each).
This reporter has tried searching other sites reporting on this, but only has found fan reaction to the news, and the actual article itself.
Source: New 'AVATAR' Comics in 2012
Written or Contributed by: Zechs