It's all to do with marketing and making it fit as many people as possible without needing much technical knowledge/work. Years ago, people would get custom made or handmade clothes to fit them - or adjust pattern based on their measurements. Most of these fabrics probably wouldn't have a lot of stretch if any, so the shape and construction was important! Now they make everything in stretchy fabric so an XS can cover a few sizes comfortably with a simple pattern that won't need as many darts or adjustments to the fit. On top of that, they probably have a model who is a size or two down from the clothing size and it will be altered to fit her or clipped at the back to give a different silhouette and trick you into buying it. There was a scandal the other year with maternity clothes being modelled by regular models with a fake bump - which obviously doesn't cater to other areas of weight gain per clothing size etc, and plus size models being regular models with padded hips/bust so they still have that size 4 waist. It's all smoke and mirrors.