Personal headcanon: after the air nomad genocide the percentage of benders dropped drastically (not just because of the Fire Nation taking captives). I think it's implicitly supported in both series.
Oo, I’ve never hd anyone sen me their headcanons before, so this is a first!
You mean just in the Fire Naton or all the nations? Possibly. Also, Bryke has apparently said that with the rise of Sozin and the Fire Nation’s conquest, the number of firebenders began to decrease, presumably because tey began to lose sight of the meaning and origin of firebending which is interesting.
Aside from that, you also have the bender genocides in the 100 Years War and the imprisoning of nonfirebending benders etc so ya. It checks out.
((that said, you’re all welcome to send me headcanons if you like.))