The tailor measures the person standing there, not the size printed elsewhere.

A standard size describes a population and fits almost nobody exactly. The tape takes the shoulders in front of it, which is the only body that will be wearing the thing. That holds for fitting anything to a person: the label is a summary, and summaries were written before you arrived. Working from the actual case saves a lot of quiet discomfort later.

Where you are arranging something for people, a rota or a room or a plan, ask whoever is in front of you rather than the category they fall into. The measurements will disagree with the average, and the measurements are right.