What exactly do you think Window metal frame and near hole reinforcement are? Structural elements perhaps?
They are structural elements
of the windows. MOST of the weight of a fuselage isn't the skin and window surrounds - it's stringers, bulkheads, keel beam, floor and floor support. I've seen airframes going through D-check with all the skin and window surrounds removed - there is still a LOT there, which makes up the lions share of the strength and weight.
The formula you so proudly point at doesn't account for all that other stuff, except in a secondary manner (yes, lighter skin means some of the other structure gets lighter, but not by 25%). While the aircraft skin is a structural element - it's
not the primary structure.