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!
waist sizes matter less because theres at least a commonly used bandaid fix for it, a belt.
Unless they are tighter than expected, especially when they shrink.
of course, hence when given the option, always buy the waist oversized rather than perfectly matching. because youll never know if it decreases.