while it does seem daft it just makes sense really, women of the same size can be massively different, mainly because of boob size, but obviously height, back width and plain old fatness play a part. If all shops conformed to the same sizes most women wouldn't be able to find something that fits properly. As it is they take slightly different body shapes for their own sizes and this leads to a variety. In some cases it will have the effect of women preferring a certain shop because they know they'll fit there, but just as many will go elsewhere for the same reason.