Think of a collection of crystals with spaces in between. Swiss cheese is a good example but not crystalline.
I'm wondering if the grain structure (crystalline lattice) on this blade is on the larger side. A fine grain simply has less available space between crystals. It would explain crumbling and the ability of the rust to penetrate deeper into the metal. The process of reducing grain structures during heat treatment may not have been well understood in the days of old razors. That kind of problem would be most attributable to overheating at some point in the forging or during heat treatment.