Pokahontas hit the nail right on the HEAD (pun alert).
Every single time I lay eyes on Brooke Kerr, Teri et al, I am wondering to myself why in the heck my curls have never and will never travel that far down my back.
Now personally, I don't care about super long length; just below shoulder length has always been great for me. (I do care a lot about thickness, but that's whole 'nother saga).
But it's always been a great mystery how people with same exact or even MUCH smaller curls pattern than mine (we're talking smaller than pencil) can have curls down their back and to their butt.
I remember asking this one girl back in high school what she did to get her hair to hang that long. She had similar curls to mine, only much more uniformly pencil-sized; no bigger, loopier ones mixed in like me. She also had much thicker hair.
She told me: "I just use my Aussie conditioner as a leave-in."
This was the early 90s and this was the first time I had ever heard that you could use regular conditioner as a leave in. Ladies and gents, I ran right over to the Walgreens and got me some Aussie 3 Minute Miracle and started heaping it on to see if it gave the curls the weight needed to hang down like her mane.
Needless to say, it didn't.
Before the past month (when I self-trimmed), I have not had a haircut in 6-7 years. And in all that time, my curls have never traveled further down my back than just below shoulder. They always, without fail, shrink up to that length.
This lead me to believe that my hair grew at a snail's pace. I told everyone "my hair just won't grow."
Now I realize that my hair had to have been growing all that time. zIt had to, right? 7 years? It just never visually manifested itself as longer.
And yes, my hair is very thin and fine when you examine it closely and recently after a wash. So maybe "density" definitely has something to do with it.
wider bigger here too, no hangtime, just growing outward and bigger by 'appearance' while loose, and only longer if stretched 
I honestly couldn't tell ya where I think it fits in the typing systems commonly used because it's not like the typical hair I see on a daily basis. It probably doesn't act like hair is "supposed" to act for X type. My great grandma would have called it 'kinky straight" and that seems a pretty good description.
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