Picture a bunch of flat ovals stacked on each other, with a slight offset.
"Picture a bunch of flat ovals stacked on each other, with a slight offset."
There is no slight offset and the ovals are symmetric. There is no way to develop curvature in the long axes naturally.
Hair consists of a bundles of keratin strands contained in dead cells. The only way to develop natural curl is if there's an asymmetric plane that gives a mirror image that can't be fixed by a rotation. That's a D shape. Cells are missing from one area of an otherwise symmetric cross section, because they are never laid down. The generating cells are missing. the papilla is asymmetric.
"I was told that curly hair was oval shaped,"
Curl is curvature of the long axis. The cells being laid consist of keratin strands, all with the same tension in the long axis. To get curvature of the length the normal tension in the cross section must be asymmetric across some plane that can't be corrected/cancelled by a rotation. Since the cells all have the same tension, some area must be missing cells. That means the papilla is asymmetric-D shaped. That D can also be distorted as an oval, or have more, or less D shape to it.