Magento2 – Google Pagespeed : any idea why it takes account of desktop css?

My question might not be magento specific, but I’m curious, as I couldn’t find the answer elsewhere (or failed to find the correct terms).

When I use Google Pagespeed on a magento 2 website with a template based on the basic one (or Luma), it says :

Remove unused CSS

…css/styles-l.min.css

But the code for the link to this CSS is :

So it’s supposed to be only use if width is more than 768px.

Any idea why Pagespeed thinks it have to be taken account ?

Thanks !