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 just learned today since I’ve never been compelled to look up any sort of conversion chart, but I had a hunch because dress/pants sizes are all over the place. One brand’s mostly-honest 12 is another brand’s flattering 4, shit like that