One can go to a website, open the chrome developer console and widen it and see the page width and height in the top left next to the developer console.

If you go in the console and run window.innerWidth to get the viewport width, it doesn't match what's shown in the top left.

In this case, innerWidth shows 1278 and the top left of the console shows 1023.

Why the mismatch, and what command is needed to get (what seems to be) the correct width, located in the top left of the console?