A shopper can wear three different sizes in three shops on the same afternoon. Clothing sizes are not measurements, and nothing requires them to mean the same thing from one label to the next.
No standard is enforced
Voluntary sizing standards have existed for decades, derived from historical body measurement surveys, and manufacturers are free to ignore them.
Because the standards are optional and the underlying population data ages, most brands develop their own size charts instead.
The label is therefore a claim about who the garment is for rather than a statement of its dimensions.
Each brand builds around a fit model
A garment begins as a block, which is a base pattern fitted on a live model of specific proportions chosen to represent the brand's intended customer.
Two brands using models with the same waist but different hip or shoulder proportions produce different garments carrying the same number.
The block is also what gives a brand a recognisable fit, which is a deliberate asset rather than an inconsistency.
Grading multiplies the difference
Other sizes are produced by scaling the base pattern, and the rules for how much each dimension grows are set by the brand.
Bodies do not scale uniformly, so grading involves judgements about how a garment should change across a size run.
Small differences in those rules compound at the ends of the range, which is why sizing tends to diverge most at the largest and smallest sizes.
Vanity sizing shifted the numbers over time
Brands discovered that customers respond positively to a smaller number on the label, and adjusted their charts accordingly.
Because each did so independently and repeatedly, a given number now corresponds to a larger garment than it did decades ago, with the shift varying by brand and market position.
Premium and mass-market labels drifted at different rates, which is part of why the gap between brands is so wide.
Returns are pushing measurements back into view
Online selling made mismatched sizing expensive, since a customer ordering several sizes and returning most of them erases the margin on the sale.
Retailers responded with published garment measurements, fit descriptions and size recommendations based on previous purchases.
The practical approach for a shopper is to ignore the letter or number and read the measurement table, which is the only figure with a fixed meaning.