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.
Yeah, the two measurements are really not enough to fit all the different boob shapes. And just offering different shapes with the same two measurements leads to problems for those who otherwise could rely on the two alone.
I have that problem with trousers where one measurement for width is not enough to fit both my waist and my hips. With bras it’s just that apparently you can’t have more than B or at maximum C if your underbust is 28/30. According to companies at least.