So we all know that there are 4 temples: North, South, East, and West. The first two have men and the last two have women. Now, we have only seen the North and South Temples. The East was mentioned but nothing has been said about the West possibly due to its location supposedly near the Fire Nation. What am I getting at? Just hear me out.
Nearly evrything in the Western Earth Kingdom is controlled by the Fire Nation, and they were only able to get the North, West, and South. The East could still be fairly safe since it's harder to reach. So maybe some Airbenders have stayed there and not shown themselves, giving the idea that they are wiped out. So unless the Fire Nation went west and landed on the eastern coast of the Earth Kingdom, the idea of at least another Airbender alive is possible.
But this is just my theory. Until they ever make it there and prove me right or wrong, it's just a theory??that I got while reading another posting of a completely different topic. ^_^