Stupid and rhetorical question: Why do women's clothing sizes have to be so confusing?!
Stupid non-rhetorical question, short form: Why do the manufacturer's measurements for my perfectly fitting dress indicate there is no way in heck it would fit me? What am I missing about the bust-waist-hip measurements system?
Long form:
I am looking on eBay for a cheap dress to as a base for a Halloween costume (a Dalek), but I'm having a hard time judging whether the dresses are likely to fit. Then I had a flash of inspiration: I have a dress bought in the last year or so that fits me perfectly and is from a well-known brand--I can look at the manufacturer's size chart and get a firmer idea of how those measurements translate to the real-life fit. When I actually checked the size chart, the bust measurement is slightly larger than what I measure and the hip measurement is a little small but not really relevant because the skirt flares starting above the hip. But the waist measurement is several inches smaller than the narrowest part of my torso (which is not my actual waist).

Unless my dress is a TARDIS, how is this possible? Anyone more experienced in buying dresses have any helpful guidance about the weird world of women's clothing sizes and how to guess from the measurements if a dress will fit?