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 would assume they’re not but that’s because I notice that depending on who makes what shoe depends on what size I have. I also see this in pants and shorts for men as well. I just assume nothing is actually standardized due to QA never really catching things.