Healthy hair grows differently because unhealthy hair breaks off more easily, so it just looks like it isn't growing as fast. Even when kids are older, if their hair is scraggly and terribly worn out looking it never seems to grow. If it is kept trimmed and healthy it looks like it grows inches each month. That's the difference.
As for curly vs straight.
As each and every one of us age our hair changes with our hormones our nutrition everything we do has an effect on our cells. When we got to a certain age the hair on our legs turns darker and is more coarse. It does not matter if it is shaved or not. It turns darker and gets more thick and noticeable over time. The older you get the worse it gets too.
DNA is what will determine her hair curl or lack of. The hair growing thicker and becoming more coarse is making her hair look straighter. The lighter the hair weight the more curl it will have. If you look at people who have curly hair, if it's long it looks more wavy, more bouncy. The people who keep their curly hair cut shorter, even in layers, then it looks curlier, lighter means more curls and more bounce.
If you want her to have bangs have bangs. She will outgrow them soon enough and want them grown out so she can wear pony tails and have all her hair off her face.
Hair can also change due to meds. A friend who did chemo lost all her hair. When it came back in it was a total different color and it was curly. She had sort of reddish brown hair and it wouldn't hold a curl unless she put a perm in it every couple of months. Her new hair was black with super curls....lol.