The same nominal size fits differently from one brand to another, and often between garments from the same brand. The inconsistency follows from how clothing is designed rather than from carelessness.
Every brand builds from its own block
Patterns start from a base block representing the body the brand designs for, and each maker develops its own block from its own measurements and assumptions.
Those blocks reflect a target customer, so a brand aimed at one demographic produces a fundamentally different shape from one aimed at another, even at identical labelled sizes.
Because the block underlies every garment in a range, a customer who fits one brand well tends to fit it consistently and to fit others unpredictably.
Fit is checked on a single person
Garments are tried on a fit model whose measurements match the brand's base size, and adjustments are made until the sample hangs as intended on that individual.
Any idiosyncrasy in that person's proportions is therefore built into the whole range, and a change of fit model can shift a brand's sizing noticeably.
Since fit is judged visually as much as by measurement, two brands using identical measurements can still arrive at different garments.
Grading spreads one size across a range
The other sizes are produced by scaling the base pattern according to a grading rule, which specifies how much each dimension grows between sizes.
Bodies do not scale uniformly, so a rule that works near the middle of a range fits less well at the extremes, where deviations accumulate.
This is why sizes at either end of a range are frequently reported as fitting poorly, even where the middle of the range is reliable.
Standards exist but are not binding
Published size standards do exist, and they specify body measurements associated with size designations, but adherence is generally voluntary.
Standards also date, since the populations they were derived from have changed, and updating them requires survey work that is expensive and infrequent.
Labels can additionally refer either to body measurements or to garment measurements, and the two conventions coexist without being distinguished on the label.
Manufacturing tolerance adds a final layer
Factories work to a permitted deviation on each measurement, so two garments of the same size from the same order can differ within the allowed range.
Fabric behaviour compounds this, because knits relax and shrink differently between batches and the same pattern produces different results in different cloth.
The practical consequence is that a size is a starting point for selection rather than a specification, which is why measurements printed on a label are more useful than the size itself.