It’s so bad that my fiancée has some bras that say she’s a B cup and others that says she’s a D cup. In order to go bra shopping, you have to actually try them on to find out if they fit.
If I had to try on underwear to see if they fit, I might not bother with underwear at all!
You can add numbers correctly, but why the hell would you do that?
Adding cup size to band size is a bit like buying pants and adding leg size to waist size. One measure cannot compensate for another.
A woman with band size 32 measures 32 inches around her torso underneath her breasts. That’s the number she needs, or else the bra is going to be too loose or too tight under the breasts.
A woman with small breasts and large chest cannot buy a bra with tighter band size that she wouldn’t be able to put around her chest and compensate with bigger cups. That would mean she would have to leave the bra open in the back and wear it as a strange kind of napkin with huge cups hanging loosely on her small breasts, leaving a large room above them.
The same goes for a woman with large breasts and thin chest. If she bought a bra with bigger band size and compensated with smaller cup size, it would be very loose around her chest and there would be two funny little cups resting on the top of her much larger breasts.
I’m taking it into caricature territory to make it easier to imagine, but even if the differences in size are not that big, it’s kind of what it actually feels like for a woman.