Why French postwar torpedoes are described as 22-inch

Why did France adopt 22-inch torpedoes after World War One?

Because the weapon was designed to a French metric dimension that English-language sources round to about 22 inches.

French engineers specified the weapon in millimeters rather than selecting an inch-based standard. The resulting diameter converts to a little more than 558 mm, which English-language accounts round into the familiar 22-inch label. British nominal 18-inch aerial torpedoes provide the reverse example because they were actually 450 mm, or about 17.72 inches. The apparent difference in national caliber choice is therefore partly a product of translating between measurement systems.