Every manufacturer in the world has a different opinion as to what those letters mean.
It’s an entirely different problem when I buy two pairs of presumably the same pair of jeans in the same style and size yet one can barely be buttoned up and the other requires a belt at all times.
Finding knit caps for my pumpkin sized head is challenging. I'd find a good fit but then couldn't reorder the same item.
I stumbled onto the notion of selecting by weight instead of the declared size. Success!
More recently, there was a HN thread about buying good jeans. I then noticed the better quality mfgs also include the fabric's weight in the item's blurb. Which I then used to inform my foraging.
The trick? The company in question is Belleville, and they make boots for the US military; they just also happen to be good hiking boots. I suspect that people in charge of logistics as DoD don't like it when things suddenly change, which would explain why they still have the same model still going for 25+ years, and why the sizes and everything else stays the same.