When M and S shirts were made in the UK, I really am that old!, a firend who worked there said they dreaded the M and S quality control inspecter coming round. Their specifications were very proscribed, stitch tension even
I can confirm that this is true - a friend of mine worked for an M&S supplier - if the spec was 8 stitches to an inch, and the randomly selected item had 8 and 1/2 - the entire batch was rejected. That is how M&S used to have a reputation for quality, and how you could rely on the sizes.
Anything seems to go now. I bout a pair of linen trousers in the summer - they were lovely and a perfect fit, so I went back and got another pair. These didn't fit properly. When I checked them there was over two inches difference in the waist sizes.