A side note, why do they say most women can use a variety of bra sizes such as 34-C being equivalent to a 36-B ? The band is around the ribs. The cup is how much the breasts protrude. So how can there be more than one "proper" size?

Could you imagine men accepting shirt sizes of 16 x 32 as being the same as 15 x 34 ? (First number neck circumference, second is length of arm.) Surely if your neck were 16 you'd be unable to button a 15, regardless of the arm length.

Why doesn't the same apply to bras? A 36 band should slide down the ribs of a women who should wear a 34. And a women who fills a C should overflow a B.

I guess I just have a simple male mind, unable to comprehend the complexity (seemingly needless complexity, to me) of female garment sizing.

Part of the reason for the variation comes from differences between styles and/or manufacturers. I've had pants that state one size, as you're aware, men's pants are usually sized by waist and inseam, but were either a little too small or too big. My mother claimed this was partly the result of where the fabric was in the pile as the big blade cut the rough pattern in the pile of fabric. I would also say it is a function of the skill of the 9-yr-old child who is sewing the garment in some sweat shop in some third-world country.

Also, every woman's body is a little different. One breast is always larger than the other (usually the left one). She might be on the line between sizes so the smaller band/larger cup might fit better or the larger band/smaller cup might fit better. Also, she might want more cleavage and lift which is better achieved with one versus the other. The fullness of her breasts might also create variance in how a particular bra fits. Finally, she might find a bra she loves, but there is no size that fits her properly, and she can't wear the style.

I guess one can think of it sort of like shoe sizes. A typical man's shoe is deigned with a D width (in the US). I wear a EEE. Each half-size increase makes that D wider. Thus, my 10.5 EEE is the same width as an 11 EE, 11.5 E or 12 D. I often end up buying regular size 12 sneakers for just normal daily activities because they are the right width even if they are a little long.

I guess one can think of it sort of like shoe sizes. A typical man's shoe is deigned with a D width (in the US). I wear a EEE. Each half-size increase makes that D wider. Thus, my 10.5 EEE is the same width as an 11 EE, 11.5 E or 12 D. I often end up buying regular size 12 sneakers for just normal daily activities because they are the right width even if they are a little long.

LOL. Pretty good analogy actually!

I'm actually an interesting case... No male shoe EVER fit me right. If they were wide enough in the pad, they were too wide in the heel, and if they weren't too wide in the heel they weren't wide enough in the pad.

I always used to wear 8.5 EE's just to keep from pinching my pads, but would tighten the laces so tight down towards the heel and up the ankle that the sides of the shoe would cross! It was the ONLY way to get mens shoes to fit my feet! Even then they were still kinda long... but just try to find a 7.5 EEEE... hah! Good luck! (And even then it still wasn't comfortable, but at least it was wearable.)

Switching to women's was the best thing I'd ever done for my feet. Now an 8.5 D / 9 B fits me perfectly. Which D is a wide for women's, C is considered a "half wide", while B is standard. Those narrows you see some precious few women wear? Those are AA's! Damned lucky cows with such tiny feet! And apparently there are even XS/AAAA!

Women's shoes don't narrow by as much as men's shoes per letter grade btw. Which explains the existance of such a huge range of sizing! I guess women are more likely to try to find that "perfect" fit?

The shoe analogy is very good. If bras sizes were specified in a more concrete way, they would confuse the heck out of shoppers. Let's take a look at it.
Band sizes are something you can play with because most bras have multiple rows of hook/eyes allowing you to adjust the actual circumference. Cup sizes are just bizarre because they try to boil down several attributes to a single letter. No kidding it doesn't work. A cup measurement which would have a hope of fitting right would have to include the circumference of the breast, the spacing of the breast gap, the shape of the breast, the projection of the breast center, and the desired shape while wearing it. The band would have to include the minimum and maximum circumferences for comfort, the depth of the chest and the height of the shoulders from the band. Unless it is a halter, crossover, t-back, racerback, or strapless. Would you rather look for a 38b, or a 36/39S9P-16WN3E (36-39" chest circumference, Shallow chest, 9" shoulder rise, Perky look - 16" breast circumference, Wide gap, Normal shape, 3" projection, Equally sized sides)?
It's more fun to shop for one you love.