The short answer
Your screen resolution is reported above in CSS pixels. The browser viewport is the usable page area inside the browser, while device pixel ratio explains how CSS pixels map to the physical pixels of a high-density display.
Screen resolution versus browser viewport
Screen resolution describes the width and height available across the device display. Viewport size describes the area currently available to the web page after browser controls, split-screen mode and window size are taken into account.
On a desktop monitor, resizing the browser changes the viewport without changing the screen. On a phone, rotating the device or opening browser controls can alter the viewport while the underlying display remains the same.
Why the number may differ from the display specification
Modern high-density displays contain more physical pixels than websites use for layout. Browsers expose CSS pixels so text and controls remain readable, then use device pixel ratio to map those layout pixels onto the hardware.
A device pixel ratio of 2 means one CSS pixel is normally rendered by a two-by-two block of physical pixels. Browser zoom, operating-system scaling and accessibility settings can also influence the values a website sees.
When this measurement is useful
Support teams use screen and viewport values to reproduce clipped menus, mobile layouts and scaling problems. Designers use them to test responsive breakpoints. The result is also one of many signals that can contribute to a browser fingerprint when combined with other device details.