Bra sizes differ in countries because of the various modes of measurement. In most European countries your band size measurement of your torso. Your cup size is also measured then the difference is what your cup size is. In the US on the other hand, bra measurements are in inches.
They measure the under the bust then add five inches to get the band size. If the total is an uneven number, it is rounded off to be an even one. The cup size is determined by the difference between the bust and band size measurements.
Other parts of the world also have various methods of measuring bra size. In France for example, you can add 15cm to your European band size to get your band size. Some countries like the US make use of double letters while others don’t. It’s important to use an international bra size converter when buying your bra so you don’t mix up the various sizes.