Check Mobile Orientation
Not all mobile devices and browsers correctly report the device orientation. According the the World Wide Web Consortium (W3C) Working Draft 1 from December 2011 "DeviceOrientation Event Specification" the orientation is given by three angles representing compass direction (α), tilt (β), and rotation (γ). The 0 values for these is with the device flat, screen up, pointed North. The W3C document has pictures showing the rotations. The table below shows how your particular device reports its orientation. The background color indicates angle response is within limits. The descrptive text is to help with orientation understanding.
In the table above the symbology [M,N) means all values from (and including) M to (but not including) N.