Why the Same US Size Doesn't Fit the Same Across Brands
Shoe size numbering standards specify a length, but not every brand's specific last (the mold a shoe is built around) or width proportions, which means two different brands' size 9, while both labeled correctly, can genuinely fit differently — actual fit within a labeled size varies more than the size conversion chart itself would suggest.
This is a genuinely common and understandably frustrating experience, and it comes down to what shoe sizing standards do and don't actually standardize.
What size numbers actually standardize
A shoe size number is primarily tied to a specific foot length (and to some extent width category) — it doesn't standardize the exact shape of the shoe's last, the volume around the toe box, the arch support structure, or other fit-relevant design choices that vary meaningfully from brand to brand and even between different shoe styles from the same brand.
Why this produces real fit variation at the same labeled size
Two shoes labeled the identical size, from two different brands (or even two different styles from the same brand), can be built around genuinely different lasts — one running narrower, another running wider or with a different toe box shape — producing real differences in how they fit despite sharing the same size label.
A practical approach given this variation
Using the size conversion chart as a reliable starting point, but trying on shoes (or checking a specific brand's own size guide and return policy) before committing, especially for a brand or style that's unfamiliar, accounts for this real fit variation that the size number alone can't fully capture.