Why does outerWith give different values for some websites?

I wonder why window.outerWith gives me a bigger value at Stackoverflow.com than at other websites in my browser. I have just noticed this behaviour only on Firefox and Stackoverflow. Shouldn’t it be the same on all websites?

In MDN Web Docs it says:

Window.outerWidth read-only property returns the width of the outside
of the browser window. It represents the width of the whole browser
window including sidebar (if expanded), window chrome and window
resizing borders/handles.

Example outerWidth at fullscreen:

  • Stackoverflow: 2070
  • Discord.com and the others in my tabs: 1863

So far I haven’t found any website that has a similar behaviour.

console.log( window.outerWidth )

steps to to reproduce it

  1. visit stackoverflow.com
  2. open Developer Tool / Console
  3. write window.outerWitdh
  4. visit google.com or other side
  5. write window.outerWitdh in the console

In Firefox these two values are different. Stackoverflow has a larger value.