Like, imagine if those sizes were equal to some actual body part measurements (or the garment's dimensions itself, whatever) in millimetres/centimetres?
Expensive clothes that are meant to be altered will usually have better tolerances, at least in my experience.
this is one of the main reasons i refuse to buy any clothing intended to be worn outdoors from amazon, shein, aliexpress, ebay, &c.
Which is exactly why I ask for sizes to be garment's actual sizes, not some made up fairy numbers (and don't even get me started on X+S|S|M|L|X+L markings).