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!
It’s been the opposite for women’s clothing in my experience. I’ve had to give up multiple brands as their shirts got too big. Same labeled size, same style, but it’s suddenly 2” wider.