I have this Avatar/Korra headcanon that your birthday affects your bending powers. Like, if you’re born in your element’s season, near the solstice/equinox, you’re better in bending. Likewise, being born in the opposite season makes your bending weaker, or at least you need to put more effort into it.
Then I added Avatar map into it, and decided when everyone’s birthday was, based on their bending prowess. So, Azula is a magnificient firebender = she’s born close to summer solstice. But since Fire Nation seems to be on the southern hemisphere, she’s born in December. Got it?