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!
And it is more complicated even than that. I am a small busted woman and yet the best fit I can get is 34D. The 34 makes sense, underbust is 33. The D is what I measure but most have too much room. I still need that size because the circumference of the boobs fits in that wire; any smaller is too narrow.
I think bras need 3 measurements not 2. I need band 34, wire size D, cup capacity closer to C. And there are plenty of women in the opposite situation too, with more projection but smaller circumference.
So the non-standardized sizing is a workaround for that problem.