The same labelled size fits differently across two shops on one street. Clothing sizes are not measurements, and nothing requires them to agree between manufacturers.
Sizes describe a fit model
Brands develop patterns around a fit model, a person whose proportions represent the customer the brand intends to dress.
Every size in a range is derived from that base pattern, so the whole range inherits the proportions of one body.
Two brands with different fit models produce genuinely different garments at the same label, because they started from different shapes rather than different measurements.
Grading multiplies the difference
Sizes above and below the base are produced by grading, which increases or decreases each measurement by set increments.
Those increments are a house decision. One brand widens a shoulder as a garment gets larger while another leaves it and adds to the body.
The result is that differences between brands grow at the extremes of a range, which is why fit becomes less predictable the further a size sits from the middle.
Standards exist but do not bind
Published sizing standards describe body measurements associated with size designations in various markets.
They are voluntary in most places, and a garment can carry any size label the manufacturer chooses without contradicting any rule.
Numbering systems also differ between regions, so conversion charts translate labels rather than guaranteeing that converted sizes fit alike.
Labels have drifted over time
Size designations have shifted so that a given number corresponds to larger measurements than the same number did decades ago.
The drift is commercial. A shopper who takes a smaller number in one shop is more likely to buy there, and competitors follow.
Because the drift is gradual and uneven, garments from different eras or different price points can differ by a full size or more at the same label.
Fabric and construction decide the rest
A stretch fabric tolerates a wide range of bodies at one size, while a woven fabric cut close fits a narrow range and nothing else.
Ease, the deliberate space between body and garment, is a design choice rather than a consequence of size, so a relaxed cut and a fitted cut built on identical body measurements are different clothes.
This is why brand-specific charts giving the garment's own measurements, rather than the body they are meant for, are the only reliable comparison between one shop and the next.