The box model: two sizing models

box-sizing: content-box (default). The width you wrote (320px) is only the content. Padding and border are ADDED, so this card renders 338px wide.

box-sizing: border-box. The width includes padding and border: this card renders exactly 320px wide.

Open dev tools and read the box-model diagram for each card — the Computed pane draws content, padding, border, and margin to scale.