This is done to fill the missing window.devicePixelRatio on the WP platform. Basically all our layout sizes are done in em units and we set font-size on the root element to a fraction of the device viewport size in order to size the UI elements automatically for the different WP devices. This works okay on devices, but backfires if you set a WP userAgent on a desktop computer - thus the problems you have there (additionally our WP theme uses some adaptive colorization in WP, so on desktop you may have color differences).
However your page should work okay in your Lumia 920. Can you share it, so that we can see what exactly happens? I can convert this post to a support ticket if you value the privacy. Thank you in advance.
Join us on our journey to create the world's most complete HTML 5 UI Framework - download Kendo UI