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.
I don't have all of my references here, but what I remember is that the 38 super is the descendent of the 38 auto which was first marked by Colt as the 1903 38 auto. The bore diameter was chose because of the legacy to the 9mm. If I remember right, the 9x19 (9 mm parabellum) was introduced in 1899 or 1902 and was the key.
Now if you look at the 38 special as .357 in the older guns you can find .358 versions. Again, from memory, the .358 was the original size of the "38 special" when loaded in lead and .357 was for jacketed.
Keep in mind that the 357magnum came along in 1935, the super was 1930(?) and the 38/44 was april of 1930.
Now can someone fact check me here?
10mm and 357 sig. The best things to come along since the 38 Super.
Actually, according to Frank Barnes in the standard reference book "Cartridges of the World" the 9mm Luger(9x19) is .355 in dia. The .38 Special and .357 Magnum are .357 and the .38 Super and it's daddy the .38 Auto are .358. However many handloaders and factorys use 9mm ammo for their .38 Super loads. It's a matter of what's available. When making cast bullets the .358 dia. is readily available and helps to make the .38 Super as accurate a round as it is while some get by with the smaller 9mm jacketed bullet and save some money.
The .38 Auto was designed by Browning, if I'm not mistaken, at Colt's request in the hopes that it would be adopted by the U.S. military in a semi-auto. At the time the service round was the .38 Long Colt with the.357 dia. The military was looking at semi autos so the .38 was it. But than they changed their mind and opted for the .45 and the 1911 was born.
Still Colt had a gun and a round and began to sell the .38s on the civilian market. In 1929 it was beefed up to become the .38 Super. From that time to about the early 80s or so the most powerful round that could be shot out of a pistol. The .357 Magnum was strictly for wheelguns.
The 38-44 is sometimes thought of as a seperate load but it is actually only a +P+ load of the .38 Special that was recommended for the Colt New Service and the S&W Heavy Duty and Outdoorsman (called the 38-44)both standard .38s built on larger N frames.