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!
That inconsistency is not exclusive to women’s undergarments… I bought a half-dozen of the same American Apparel hoodie in different colors… very different fits.