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!
I think this is filed in my head under “no two manufacturers’ sizes are the same (even if they’re supposed to be to a standard)”, and “this is especially true of women’s wear”, so while I may have known about bra cup sizes specifically at some point in the past, I’m not sure I did at the time I arrived at this post, and yet am thoroughly unsurprised to (re)discover it.