Some what similar situations exist in other caliber "familes", and basically you need to learn what is the "norm" and what are the exceptions.
I'll name a few, there are many
Today, the .22 centerfire standard is .224" .22 Hornet, .218 Bee, .219 Zipper, .220 Swift, .221 Fireball, .222 Rem, .222 Rem Mag, .223 Rem, .225 Win and .22-250 all use .224" bullets. Pre WWII .22 Hornets may have .223" barrels. The .22 Rem Jet uses a .222" bullet and the .22 Savage Hi Power uses a .228" bullet.
Most of the US and commercial.30 cals use a .308 bullet, some European ones use the larger .311 bullet. (.303 Brit, 7.62 Russian, both x54R and x39, and the 7.7x58mm the Japanese used in WWII.)
Some of the 6.5s use the same size bullet (.264") a couple use a different size.
There are a couple of different bore sizes that are called 9mm.
Our .38 pistol bullets come in a couple different sizes and all are close to .36 caliber, not .38. That's a result of the history of cartridge construction. Same reason our .44s are actually .43s (.429")
Our .45s are .45s, but can be .451" or .454" depending on what and when made. Pre WWII .45 Colt guns are normally .454". Post WWII they have generally become .451" to better shoot .45ACP (.451") bullets.
8mm Mauser comes in two sizes, sort of. .318" originally, then later .323".
Usually the first number in the caliber is the one you can trust, but there is an exception to even that rule, the .38-40. It uses a .401" bullet, and so ought to have been called the .40-40, but the makers didn't do that.
Many reloading manuals have a brief history of the rounds, and you can find a lot of still useful information about older rounds in outdated copies of Cartridges of the World (Barnes) which can often be found cheap in the booksellers and flea markets, gun shows, etc.
One word of warning, the older copies of COW have some good information, and some that isn't so good, and the author's opinions, which also, aren't always good.