it's just like Sokka was saying in the courtroom. some benders just have unique abilities. nobody else but Combustion Man has ever been shwon to shoot explosions out of his head.
I still don't understand how Yakone's family can bloodbend without the full moon. Is that because they are extremely powerful?
the example with metalbending is even better. metalbending was supposed to be impossible for a long time, just like bloodbending without the full moon is supposed to be impossible. but then once a particularly skilled bender happens to come across the right technique, it's discovered that it actually is possible. all earthbenders have the potential to metalbend apparently, but only with the right training (there are plenty of metalbending police in the city, but Bolin can't metal bend because he just never got the right training). bloodbending without a full moon is the same. Yakone, like Toph, was just a highly skilled bender who happened upon the right technique. all waterbenders have the potential to do it if they get the right training.