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"...the 9mm bullet is the same as the .38 Special..." No it isn't. A jacketed 9mm is .355", cast .356". The .38 Special and .357 Mag use a .357" bullet.
"...how the numbers for the caliber are decided..." Usually by the marketing types. Sometimes it's to avoid confusion between cartridges. For example, the .38 Special case is 1/10" shorter than the .357 Mag. Other than that they're identical.
'Calibre' is a measurement of the inner diameter of a tube. In a rifle barrel, the bore is the size of the hole without the rifling. In North America the calibre is measured from groove to groove. European cartridges are metric and describe the bullet diameter plus the length of the case in mm's. As in 7.62mm x 51mm. (It's daft too as the .308" bullet used doesn't convert to 7.62mm's. It converts to 7.82mm.) Some European cartridges are named for the bore diameter and have nothing to do with the actual diameter of the bullet. As in the .303 British. The bullet measures .311" to .314". That'd be the groove diameter.
As you can see, trying to figure out why a particular cartridge is called what it is can make you crazy.
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