There is no hump when spun on a lathe because it's still equal distance from center to od at all points when measured perpendicular to center. But there is a hump when you run your hand up and down it on the long axis.Like I said, this concept must be to advanced for you to grasp. This isn’t some abstract theory like you claim. A cone has a standard rate of increase. What happens when you vary that rate of increase? Up or down. You'll never get a smaller section of the butt. It never becomes smaller like you claim. Its always increasing. But because it no longer matches the rate of increase of the straight line, you’ve taken it completely out of context. It’s not smaller diameter for fucks sake. Nothing tapers back down. It’s just no longer doing what you think it should be doing. Again, it’s all due to the rate of increase going down below what it was. That’s what creates a ‘hump’ when rolled. But when spun on a lathe, there is no hump.
If you cut it as you describe it's not a curve it's just a taper that decreases in a non linear fashion. But it's always decreasing.
This is a curve ) Here is another one ( Tell me how you put this in a stick of wood without it being smaller at some point than previously and reach the end of the stick?