A bit of a moan

Ok......so many years ago I worked at a manufacturing company who contracted for M&S.......think back in the day when clothing WAS made in Britain.
I know for a fact that if an order for certain garments in specific sizes could not be met then lower or higher sizes of the identical garments were relabelled! Sneaky and underhand but it does explain the inconsistency in sizes.

I go to Bon Marche for their sizing of nether garments which is consistently accurate and easy to understand; They also have uncomplicated distance buying with easy returns.

I buy trousers from Lands End. There sizing is consistent.

What I find is highly erratic is 'description' sizing - small, medium, large. I am a size 10/12 and in most cases this equates with 'small', but I was ordering a clothing present for DGD this weak - who is a size 12, so I clicked on 'small', then some guardian angel of my shoulder, whispered 'check' so I did and the company involved 'descrption sizing went: Size 6, very small; size 8, small; size 10, medium; size 12, large!!
So I have ordered her a large size sweater!!

I just go to the shop and try them on. Saves a lot of time and moaning!

I don't know why they bothering sizing anything. Sizes are always inconsistent.

If I order online I always order more than one size but, as OP says, even within sizes items can differ.

I found this with m & s jeans particularly , I take several size 12 in changing room and they all vary!!! The same with Next jeans too! I’ve given up ordering them on line due to this reason.

I noticed this a few a few years ago from M&S but with their slippers. I am convinced the stores go to the cheapest supplier to cut costs. Therefore the end result is that the goods are not such quality and materials are shoddy.