As someone who worked in the industry for more years than I care to remember & have been involved with setting up several of the large groups sizing policies, I can say with absolute confidence..........YANBU,
& even with the smaller more "fashioniista" chains officially it should be
S=8-10
M=10-12
L=14-16
That said, a lot of the smaller so called designer boutiques are often selling whats known in the trade as "cabbage"........ie: factory over makes, garments that the factory themselves have made from Fabric they have managed to pinch back from an order placed by a high street store supplier,
also, on most garment patterns, on standard width fabric you will be able to fit, 1 sise 8, 1 x10, & 1 x14 across the width, meaning that those sizes are much cheaper to produce,.........they are then labelled s, m, l, & sold in wholesale shops, usually to small boutiques & market stalls.......these are usually run by people who have no idea of what sizing should be, so the S,M,L labels mean they don't return the goods & complain the sizing is wrong.
That said, as you know even the high street chains vary massively, & those aimed at teens do tend to have a smaller fit....daft really, as teens are much bigger these days than they used to be ....particularly daft, as one of the big ones I used to work with, their house/fit model was in her mid 40s.
Mind you...I collect vintage patterns.....now THAT is scary......I'm a size 12, not even a big 12.....but back in the 40s, according to my old patterns, I'm a size 20+