It's all genetics, baby.

But I've noticed something with my hair... it is 2b/2c, sometimes even 3a. And it all depends on how I wash it. If I use any harsh shampoos, it kills my curls. Which is why a while back I threw out anything with 2 or more harsh chemicals in the ingredients. They look like depressing, ugly waves when I wash with harsh chemicals.

When I only wash with conditioner, my curls start halfway through after some good healthy looking waves.

And when I wash only with herbal tea, all of my hair is curly(well, starting about 3 inches off of my scalp).

I'm assuming that my hair isn't the only hair that acts like this based on washing. So, try out some different things and see what your own thing is for controlling your hair texture.

And I'm no expert. Just putting that out there.