There's a certain amount of consistency and standardization that customers expect from a retailer, and size is one of them (perhaps the third most important after quality and customer service). I work with measurements every day, and I've done a lot of research to come up with my size chart (which works well for many Indian women), so I'm not representative of an average shopper. However, I certainly can understand the frustration (and confusion) in having to try on different sizes for different styles all in the same store.
So what's the deal here? Why can't us designers get it right? Well here's the thing: we're under pressure to provide a variety of silhouettes to suit different body types, but our size chart remains the same because we too crave a certain consistency.
Let me illustrate. Take a look at this shirt style below called Casual Thursday.
Casual Thursday is a a shapely, fitted shirt and the model is wearing a size 2. If a woman who has a bust measurement that's an inch larger than a size 2 tried this same shirt, she'd find it very hard to move her arms. A style like Casual Thursday looks great on women who are evenly proportioned (doesn't mean then all have to be a size 2, just that their proportions need to be similar to our standard sizes).
Now look at this draped style, aptly called Dolman Drape.
This silhouette is designed to be worn loose at the bust and fitted at the low waist area. It's the same size 2 model, and you can see how this top adds volume to her torso, making her hips look narrow in comparison. This style works well for women who are flatter at the bust and/ or wide at the hip. If a woman who had a bust measurement of size 4 wanted to try on a Dolman Drape in size 2, she would have no problem because it's loose around the bust anyway. In fact, even a woman with a size 6 bust could wear this silhouette, if her waist and hip measurements were closer to a size 2.
Now our final silhouette for this case study: Draped Shirt.
Draped Shirt is tailored around the bust, but loose around the waist. The model is wearing a size 2, but let's say a woman who had a bust that corresponds to a size 2 and a waist that was closer to a size 4 tried this on, she'd have no problem carrying it off.
A customer who has a bust of 35" and a waist of 26" would wear a size 2 in Dolman Drape, but a size 6 in Draped Shirt.
So long story short: our size chart is standard, but each silhouette has a different shape. You should definitely be suspicious if you shop at a store where all the clothes that fit you have the same silhouette but different size labels. However if different silhouettes that fit you have different size labels, that's just because your body measurements can't be categorized easily into that brand's standard size chart (not a bad thing- it just means you're unique)!
And in case you're worried that this is all so complicated, look on the bright side: once you figure out your body proportions, you'll be in a much better position to assess which silhouettes flatter you the most!