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!
When going for brands, like Lounge Underwear or even more expensive ones like Marlies Dekkers, Aubade or Lise Charmel (instead of cheap imports from China) there’s almost never a problem with the size. There have been maybe 2 or 3 bras (cheap ones) that did not fit in the last twelve years and everything else has the same size and fits her.