I don't know what it's like where you are, but I was told, when I went for my bra fitting at a specialist lingerie shop, that big-name chain Marks and Spencer tend to measure too big round the back and too small in the cup when they do their fittings (I was told and Marks and Spencer that I was 36D when I'm actually 34F.)

And when you get that sort of information from people who supposedly know how to fit bras correctly, it's hardly surprising that so many women are wearing the wrong bra size.