A shopper who wears one size in a running shoe often needs a different number in a court shoe from the same company. The size on the box is not a measurement of anything standardized.
The number refers to a mold, not a foot
Shoes are built around a solid form that determines internal length, width and volume. Each shoe family uses its own form, developed for a particular use and shape philosophy.
The size printed on the label is the name that form carries in the factory. It is a production reference that reached the consumer by convention.
Two forms named the same size can differ by several millimeters of internal length. Neither is wrong, because there is no external standard to be wrong against.
American sizing carries its own history
The scale used in the United States descends from older trade practices rather than from a metric definition. Its increments are fixed, but the starting point and the width grading vary by manufacturer.
Men's and women's scales are offset from each other by a conventional amount, which is why unisex models list two numbers. The offset is a convention, not a physical relationship.
Imported footwear adds another translation, since European and Japanese scales convert imperfectly. Conversion charts round, and rounding lands differently in each direction.
Use case changes the intended fit
A distance running shoe is designed with room at the front because feet swell over miles. A basketball or lifting shoe is designed to hold the foot still.
The same wearer therefore needs different internal lengths for those purposes, and brands build that difference into the form rather than expecting the customer to adjust.
This is why size advice given for one model transfers badly to another. The advice describes a fit intention, not a number.
Materials move the fit after purchase
Knit uppers stretch across the width under use, while molded synthetic panels hold their shape. Two shoes measuring identically in the box will not feel identical after a month.
Foam midsoles compress unevenly under body weight, changing how the foot sits inside the shoe. Fit assessed on day one predicts less than most buyers assume.
Some brands anticipate the change and cut tighter on purpose. Others do not, and the shoe loosens beyond its intended fit.
What actually helps in a store
Measuring foot length and width on a gauge gives a starting point but not an answer. The useful step is trying the specific model, late in the day, with the socks intended for it.
Retailers that keep half sizes and multiple widths solve more fit problems than any conversion chart does. Depth of stock is the practical remedy for an unstandardized scale.
Online, the reliable signal is a brand's own guidance for that model, since it is the only party that knows the form. Cross-brand comparisons are guesses dressed as data.