Game draws outside letterbox after auto-scaling to full screen


I followed this post to add auto-scaling function to my MonoGame WP8 project (an XNA port):

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! =)