You are a different size in every brand because clothing sizes are not universal. Brands use different fit models, measurement ranges, garment cuts, fabrics and regional labels.
High-level answer
The best way to choose a size is to compare your actual measurements against the brand, garment type and fit. Size labels are useful shortcuts, but measurements are more reliable.
High-level answer
You are a different size in every brand because clothing sizes are not universal. Brands use different fit models, measurement ranges, garment cuts, fabrics and regional labels. A UK 10, US 6 or EU 38 can fit differently depending on whether the item is fitted, relaxed, stretchy, structured or oversized.
The best way to choose a size is to compare your actual measurements against the brand, garment type and fit. Size labels are useful shortcuts, but measurements are more reliable.
Why clothing sizes are not universal
Clothing size labels look precise, but they are not fixed measurements. A medium in one brand can feel like a small in another. A UK 12 in jeans may not fit like a UK 12 dress. Even two items from the same brand can fit differently if one is stretch denim and the other is rigid cotton.
Brands build clothes around their own assumptions. They choose their own fit models, grading rules and customer profiles. That is why your usual size is only a starting point.
This is also why a clothing size calculator works best when it considers three things together: your measurements, the brand, and the garment category.
The main reasons your size changes
Brands use different fit models
A fit model is the body shape a brand uses when developing garments. If one brand designs around a narrower shoulder, lower hip, shorter inseam or straighter waist-to-hip ratio, the same size label can feel different.
Garments are cut differently
A fitted blazer and a relaxed shirt are not meant to fit the same way. A cropped jacket, wide-leg trouser or oversized sweatshirt may all use the same size label but sit differently on the body.
Fabric changes the fit
Stretch fabric can make a smaller size feel wearable. Rigid fabric can make the same measurement feel tight. Knitwear, denim, tailoring, cotton poplin and technical sportswear all behave differently.
Regional sizing systems are only rough guides
UK, US and EU sizing systems do not convert perfectly. A conversion chart can help, but it cannot tell you how a specific Zara dress, H&M trouser or ASOS jacket will fit.
What to do before buying
Use your size label last, not first.
Start with waist and hips for jeans, trousers and skirts. Use bust or chest for tops, dresses, shirts and jackets. Use inseam for jeans and trousers. Use foot length for shoes. Use shoulders and sleeve length for jackets and shirts.
Then compare those numbers against the specific brand and product category.
Bottom line
Your size changes between brands because each brand defines fit differently. The label is only one clue. Your measurements, the garment type and the fabric tell you much more.