A person can wear three different sizes in three shops on the same afternoon. The inconsistency is not a manufacturing failure but the predictable result of an unregulated label.
There is no binding size standard
Voluntary sizing standards exist and are widely ignored, because no authority requires a garment marked with a number to correspond to particular body measurements.
Each brand builds its own size chart from a fit model whose proportions represent its intended customer, and grades sizes up and down from that single body.
Two brands targeting different customers therefore produce genuinely different garments under the same number, and both are labelled correctly by their own system.
Grading multiplies small differences
Moving between sizes involves adding fixed increments to each measurement, and the increments a brand chooses determine how the fit changes across the range.
Bodies do not change proportionally, so a garment graded from a smaller base fits the extremes of its range worse than the middle.
This is why a brand can fit one person perfectly and fit their friend badly while both wear what the label says is their size.
Labels drifted to flatter the customer
Retailers observed that shoppers respond positively to a smaller number, and adjusting the label costs nothing while changing the pattern costs a great deal.
Over decades this produced a steady drift, so the measurements associated with a given number have grown while the number stayed still.
The drift is uneven across brands and price points, which adds another layer of inconsistency on top of the differing base patterns.
Manufacturing tolerance adds real variation
Garments are cut in stacked layers and sewn by hand, and factories work to a permitted tolerance on each measurement rather than an exact figure.
Fabric behaves differently between batches, particularly with knits and anything containing stretch fibres, so two identical items can measure differently.
Production is also spread across multiple factories for a single style, and each has its own consistency.
Online retail turned the problem into a cost
Shoppers who cannot try on garments order several sizes and return most, and returns processing is expensive and often ends with the item discounted or discarded.
Retailers respond with detailed measurement charts, fit prediction from purchase history, and reviews reporting whether an item runs large or small.
Those are all workarounds for a label that no longer carries information, and none of them fixes the underlying absence of a standard.