I found an issue regarding to determining device based on screen width.
We divide screen width on device pixel ratio.

var physicalPixelWidth = window.screen.width,
  pixelRatio = window.devicePixelRatio || 1,
  cssPixelWidth = physicalPixelWidth / pixelRatio;

It's mistakenly because window.screen.width already has density independent pixels value (tested on several real devices).

So I suggest to eliminate this division.

Actually it'll be good to use smallest-width qualifier approach instead - the same as android use to determine small/large screen devices.