Valid question! The acp is the original designation. It stands for automatic Colt pistol. From way back in the days of John Browning when Colt was the premier pistol manufacturer. So when Browning invented the .25, the .32, the .380, and the .45, the Colt company was one of the main sponsors and makers of pistols in those calibers. Naturally, the ammo was named that way.
So acp has hung on but we just call it auto for short.
Now 9mm is a little different having started in Europe for the Luger pistol. So it's often referred to a 9mm Luger.
I think that's correct. Tell me if otherwise.
Cheers,
og