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’m between a medium and a large. More often then not though, I need the shoulder width and arm length of a large, but the cut in the torso of a medium.