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?
This might not really answer your question, but I'll give it a try.
Most size charts, but not all, give the measurements for the person who will fit in the garment, not the garment itself. All clothing is designed with "ease" built in--ease is extra room to allow the garment to fit and hang correctly, and to allow the wearer to be able to move easily in the garment. If the chart is giving the exact measurements of the garment, you would need to be at least an inch smaller than the measurements to fit into that piece of clothing.
Different types of clothing have different amounts of ease--skinny jeans don't have much, a flowing skirt has tons. A strapless dress won't have much ease at all in the bodice, because if it did, the top might fall down. But there could be lots of ease in the skirt if it is a full skirt. Or not much ease, if it is a straight skirt. Pencil skirts have a little bit of ease, but the reason most of them have a small slit or kick pleat in the back is that there isn't enough ease built into the skirt to allow for normal walking, so the slit is added so you can have enough room to move your legs normally.
So for many garments, you can be an inch or two larger or smaller than the manufacturer measurements and still have a piece of clothing that fits and flatters. Especially if the clothing is sized Small, Medium, and Large, where Small is usually two sizes, say 4 and 6, and Medium covers two sizes, 8 and 10, and Large is for both 12 and 14. But even something with a specific size, like size 8, has a little wiggle room in the fit.
The style of the garment has a lot to do with fit, as well. The manufacturer may give a waist measurement, but in an A-line dress that does not have a fitted waistline, it doesn't mean much, as the dress is not designed to fit closely at the waist, or at the hip, for that matter. The bust measurement may be the only one that really matters.
