To quote from "Pistols, Revolvers and Ammunition" by Josserand & Stevenson:
Some 200 or 300 various handgun cartridges have been developed and manufactured over the past century, and one would expect a logical, coherent system for naming them. There have in fact been several systems, but these have largely faded in favor of ballyhoo and razzle-dazzle, with the result that the pitiable amateur has no choice but to memorize every last cartridge and its corresponding designation, one by one. There is no other way.
Take these for instance: .32-20, .38-.44, .380/200, .38/.45. The first is a .32 caliber projectile loaded ahead of 20 grains of black powder or an arbitrary charge of smokeless powder to achieve equivalent ballistics. The second is an overpowered .38 Special intended to be fired only in revolvers built on the larger .44 frame. The third is a .38 or .380 caliber cartridge with a 200 grain projectile. This is a British service round, and nothing in the name tells you it is identical to our .38 S&W Super Police. Finally, the .38/.45 is a .45 ACP case necked down for a .357 projectile, which except in the case of the .357 Magnum, are always called .38s for a rather obscure reason.
....it continues on, but the point seems to be made.