Game draws outside letterbox after auto-scaling to full screen

Hi,

I followed this post to add auto-scaling function to my MonoGame WP8 project (an XNA port):
http://developer.nokia.com/community/wiki/Auto-Scaling_WVGA_XNA_Games_to_WXGA_%26_720P_with_MonoGame_for_WP8

The auto-scaling result looks good, except that the game is drawing outside the scaled-up 800x480 letterbox to its right side on a 720p screen (the left side is a black rectangle as expected). My guess is that it has something to do with the BackBuffer size… Can someone shed a light on this issue? Thanks in advance! =)