A single size label means different measurements at different retailers, and the variation is not an error. Sizing is a commercial decision made independently by each brand.
There is no binding standard
National sizing standards exist in several countries, but they are generally voluntary reference documents rather than requirements a manufacturer must meet.
Brands are free to define their own size charts, and most do, because their charts are built around the customers they actually sell to.
The number on the label therefore describes a position within one brand's range rather than a fixed measurement of the garment.
Everything starts with a fit model
A brand selects a person whose proportions represent its target customer and fits the base size of every garment on that individual.
Larger and smaller sizes are then generated by grading, which scales the pattern by set increments across chest, waist, hip and length.
Because two brands choose different fit models and different grading rules, garments diverge most at the extremes of the range, where the accumulated increments differ most.
Block patterns carry a house shape
Each brand maintains basic patterns, or blocks, from which garments are developed. These encode the shoulder slope, armhole depth and body ease the brand prefers.
A relaxed block and a tailored block can produce the same nominal size with several centimetres of difference in actual circumference.
Because customers return items that do not fit, brands have a strong incentive to keep their blocks consistent internally even while diverging from everyone else.
Sizes drift over time
Brands periodically revise their charts, generally in the direction of assigning a smaller number to the same measurement.
The commercial logic is straightforward, since a customer who fits a smaller label is more likely to complete the purchase and less likely to shop elsewhere.
The consequence is that a size from an old garment is not a reliable guide to the same brand's current production, let alone anyone else's.
Manufacturing adds its own variation
A single style is often produced across several factories, and fabric behaviour, cutting tolerance and shrinkage differ between them.
Knitted fabrics vary more than woven ones, since the yarn tension and finishing process both affect the final dimensions of the panel.
Tolerances are specified, but a garment within tolerance at one end of the allowed range still fits differently from one at the other, which is why two identical items can feel unlike each other.