Someone will probably follow with a better reason, but the only thing that comes to mind is that the 38 super was a hot rod version of the anemic, and older, 38ACP. The logic behind bullet diameters is confusing. The so called 38's range from .355 for the 38 super, through the .357 38 spl, the .360 38 S&W, and the .401(I think) 38WCF. Some of these strange combinations may have come about via the evolution of cartridges from the heeled style bullets.