I think it's the styling methods used and her haircut. Before her big chop, she had a bad hair cut. She always had slightly frizzy, pyramid hair. Once her hair began to grow out, it looked as if she had a cut that worked with her curls and she used better products.
I think you're right, my hair used to be as tightly curled as Keri's when i was younger but after years of brushing, hairdye and cutting my hair in different styles, it changed a lot. It has changed a lot after i went CG too, my curls are finally starting to come back.
It defenitely looks like she had started the CG method too, her hair looks a lot more shiny and healthy with much less frizz.
It's the volume that's been reduced after her big chop.
But her hair looks a lot more healthy now than it did in season one for instance.

Hopefully she'll start wearing her curls more often again, she's an amazing actress with a great personality and a beautiful complexion, it's a shame the stylist (or maybe just she) is trying to change that.
Hairtype: 3A/3B
Co-Wash: Enliven (Raspberry)
Conditioning: Nubian Heritage Conditioner, Enliven , KCKT
Leave In: Shea Butter Leave In, Mixed Chicks, (trying) GDLI
Styling: L'Oreal Invisi FX gel, Blended Cutie Down and Out Styles,Jessicurl RR & CCSS, CK, (trying) Pink Boots & KY(waiting for CK to arrive)
Currently: Experimenting.