i think also what happens is that there's huge variation on how the fit comes up against those size charts. even at m&s, you'll get two pairs of trousers, ostensibly the same size, that will be inches different in fit. Look at a store that uses cheaper suppliers and that will happen more often. Go right down the scale to Primark or H&M and there's no continuity at all: they're paying pennies on the item, they come with the size labels already in, and their margins can't tolerate getting someone from quality assurance to check sizes when they come in and re-label if needed.