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!
It was a long time ago that I realized that women’s clothing sizing was largely fiction. Trying to buy clothing for a girlfriend or (later) my wife based on the tag of something they already owned was an exercise in futility.