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!
Most of the bras that my girlfriend gets fits on her first try, although she does tend to prefer sister sizes over her real size. If your girlfriend is having issues with bras fitting, it might be worthwhile to read up on how bra sizes are actually calculated and do a measurement yourself. Funny enough, most girls don’t seem to know how the bra size system works either and they just get their sizes through trial and error, which seems like what has happened here.
The letter by itself is fundamentally meaningless. A 32D is equivalent to a 34B! And most girls severely underestimate their actual size. What would colloquially be called a B or C is actually an E