Why G3 and Iowa look similar on paper

How could the G3 appear as fast and well armored as the Iowa on similar tonnage despite being twenty years older?

Their hull dimensions, displacement, and intended high-speed capital-ship roles were unusually similar, while some G3 performance remained estimated rather than proven.

The two designs had remarkably similar displacement, length, beam, draft, and expected speed. G3's 33-knot figure may have been optimistic because the design was never completed and tested. G3 sits near the fast-battleship boundary from the battlecruiser side, while Iowa approaches the same boundary from the fast-battleship side. Their similar high-speed capital-ship roles therefore produced similar operating requirements despite the twenty-year age gap.