Keynote letterboxing
I am using the second display for a projector and when I just project the extended desktop it fills the screen. However with Keynote it is letterboxed (black borders left and right). But it's not just a problem with Keynote - the same thing happens with Mac Powerpoint. Strange thing is this has only just started happening (within the last couple of months) but I can't put my finger on what I changed or updated.
MacBook Pro with Retina display, OS X Mountain Lion