I'm amazed how all that works. Waist size, for example, is pretty straight forward. You're either a 32" or a 34", perhaps a 33", but pretty standard stuff. Bra size involves two metrics, cup size and measurement around the chest (I think anyway). That all seems straight forward as well, but it doesn't appear to be. The most crazy sizing (in my humble opinion) is dress size. You can be a 1, 2, 3 - even a zero, but those numbers don't correspond to anything else .... but I digress.
Uh, shoe sizes can be just as inscrutable.
Many women do not understand how to accurately size a bra. I used to be one of them. My advice is to get professionally fitted.