A garment labelled the same size fits differently from one brand to the next, and often from one product line within a brand. The label describes a company's own pattern, not a shared measurement.
The number is not a measurement
Sizes for many garments began as approximate references and drifted into arbitrary codes. The printed number rarely corresponds to any dimension of the body it is meant to describe.
Where measurements are printed directly, such as a collar or a waist in centimetres, consistency improves noticeably because the label commits to something checkable.
Everywhere else, the brand is free to attach any label to any pattern, and nothing in the supply chain forces two companies to agree.
Every brand fits to its own body
Patterns are developed on a fit model, a real person whose proportions define the base size before the range is graded up and down from it.
Two brands choosing different fit models produce different garments at identical labels, because the base shape differs in shoulder slope, torso length and hip-to-waist ratio.
Grading compounds the divergence. Scaling a pattern to larger and smaller sizes requires assumptions about how proportions change, and those assumptions are proprietary choices.
Labelling responds to how people shop
Brands know that a customer who fits a smaller label is more likely to buy, so the same body has gradually come to fit lower numbers over the decades.
The drift is not coordinated. Each brand adjusts against its own history, which is why old garments and new ones with the same label differ in fit.
The effect is strongest where the number is furthest from an actual measurement, and weakest in categories that quote dimensions the buyer can verify.
Fabric changes the fit after the pattern
A pattern is cut in a specific fabric, and the same shape behaves differently in a stiff woven cloth than in one with stretch fibres.
Stretch widens the range of bodies a single size can accommodate, which lets brands offer fewer sizes while claiming a broader fit.
It also hides pattern differences. Two garments that fit similarly when new can diverge sharply once the elastic component relaxes with wear and washing.
Manufacturing tolerance adds the last variation
Factories work to a permitted deviation on each measurement, so two garments from the same production run are not identical.
Cutting many layers at once, fabric relaxation and shrinkage after finishing all contribute, and the accepted tolerance is wider than most shoppers assume.
This is why trying two units of the same item can be worthwhile, and why online return rates in clothing remain far above those of almost every other category.