Long ago, in a galaxy far, far away.......
There was a ball and cap revolver (Colt Navy) that had a .36 bore (actually, about .374). When a conversion was made to allow those to fire metallic cartridges, they were .36 caliber bullets, but were "heel seated" bullets- bullet gripped only at the base, by a case the same size as the bullet.
The next generation of revolvers made to fire metallic cartridges used a bullet loaded inside the case. This meant the case had to be larger than the bullet. A .36 bullet had a .38 cartridge case.
After a few evolutions, that became the .38 Long Colt. While became the .38 Special. Which in 1935 reverted to it's actual dimensions when the .357 Magnum was named.
There were reports that S&W was concerned that their new, really hot stuff round would be compared to the fairly weak .36 Navy ball & cap revolver.
So- why is a 45 ACP not 45, nor a 44 magnum 44? PS- the .50 BMG does not fire a .50 diameter bullet.