When I was a boy there were two factories in our town which made shirts; mum worked in one and dad in the other. To complicate matters mum's factory also made shirts for M&S. Think of the collar on a man's shirt - there is the bit that the button and buttonhole are on and the bit that folds over. One lot measured the distance from button to buttonhole, another the distance from the edge of the cloth with the button on to the edge nearest to the buttonhole and the third the distance between the points where the fold-over bit of the collar met the shirt. So three measurements for the same size of shirt.
The same can be true of the waistband of trousers - do you measure from button to buttonhole or the full length of cloth? On top of that, different sewing machinists can take a slightly different turning, so the same size cut cloth can end up as different size garments with different machinists.
For that reason it is always wisest to try on clothes before you buy.