The band size now is just a number to help you locate the correct size within a make and model of bra, same as a dress size is. I take a dress 14 in most (not all) brands but no part of my clothing is 14" big.
The origin of the band size number was "overbust minus four inches", with the underbust assumed to be 4" less than the overbust for all women. There were no cup sizes back then, the letters came later on. What we now call "A cup" was the only cup size available.
Variations in bra cuts, improvements to construction methods, and changes to fabric used (in particular how stretchy the wing fabric is) have eroded the link between underbust measurement and band size. Even so, as a measured 27" underbust, I cannot fasten a 26" or 28" band, but can fasten a 30" painfully, find a 34" will ride up, and am comfortable in a 32" at Marksies.