Go look at Sailcut. That is a good example of "luff curve defines amount of depth, broadseaming defines position of depth"
Increase mast bend, and the extra cloth in the luff curve is sucked out of the volume the sail makes. Straighten the mast and the sail gets extra volume.
You have the same effect in jibs if you have a S curve in the luff.
If you play with sailcut you can easily see that the developed panels you cut and their luff curve is wildly different from the curve you specified. This extra material in the panels is the cloth added to give the correct 3D share of the sail.