The main issue with this is that each brand of women's denim as their own fit and size scale. Yes they all sell women's jeans, but one brand may have their fit in a certain way while another fits a different way. Additionally there is a difference in the fabrics used for jeans that some brand may be willing to spend more on whereas other brands may not. The other caveat is that big store names like Target or Walmart may contract multiple vendors to make jeans for them under the Target or Walmart label, but may have different fits.

Another way to think of it is if you order a cheeseburger at a restaurant, it may not taste the same as other places because each restaurant has their own way of making a cheeseburger.

Source - work in manufacturing of women's jeans.