When building two identical frames with different stiffness targets, the amount and orientation of fibres required to achieve the target stiffness will differ. This can create two frames with the same static weight and balance but with different swingweights.
The inertia of a physical object is the sum of all the mass in the racket times the square of the mass' distance from the COM. SwingWeight is the inertia of a racket using a 10 cm axis and not the COM. Stiffness is a measure of flexibility of the racket.