The reasons a familiar EU, US or UK label can fit differently across brands.
Start with a known reference
For why size charts differ by brand and model, the best first input is a shoe that already fits well or a fresh foot-length measurement in CM. A familiar EU, US or UK label is useful, but it becomes safer when it is tied to a real brand chart row.
Use CM before label shortcuts
CM is closer to foot length than EU, US or UK labels, so it is the strongest bridge between brands. If CM is missing, compare the closest available row and treat the result as approximate.
Check brand and model caveats
The same size label can feel different when the last shape, upper material, padding or intended use changes. Running shoes, skate shoes and lifestyle sneakers should not be treated as one fit category.
Use Sizetab for the next step
After you understand the sizing rule, open the relevant brand page or comparison page. That gives you the concrete row, related chart notes and links to the next checks before checkout.
Concrete example
If your current shoe fits at one brand and you want to buy another brand, do not rely on the label alone. Open the comparison page, match the closest CM row, then confirm gender line, width and model notes.
FAQ
Which size system should I trust first?
Use CM first when it is available, then confirm the label in the correct brand and gender line.
Can a chart guarantee fit?
No. A chart gives the best size starting point, but model shape, width, material and fit preference still matter.