Originally posted on: http://geekswithblogs.net/ihaynes/archive/2013/07/01/responsive-design-media-query-fix-for-ie10-on--windows.aspx
The version of IE10 on Windows Phone 8 apparently has a bug which results in media queries not seeing the correct device width.
This post from Devhammer explains all.
http://devhammer.net/responsive-design-fix-for-windows-phone-8-device-adaptation
I'd not noticed this on the WP8 Emulator which proves yet again that testing on real devices is essential.