Can you please explain the Net Free Area differences between ShingleVent II 12-inch and ShingleVent II 9-inch wide.

ShingleVent II 12-inch wide has more NFA than ShingleVent II 9-inch wide.

ShingleVent II 12-inch wide has more net free area (18 square inches per linear foot) than SVII 9-inch wide (6 square inches per linear foot) because the height of the ribs under the vent is taller. NOTE: NFA of a ridge vent is not determined by the vent's width but instead by the vent's height.