Ctrl+0 does not fit to screen in Photoshop CS6 13.0.1. It enlarges the image, but not nearly to fill available space. (Palettes are all on second monitor.) Win7
I found that turning off certain panels can make the difference...in my case it was the Timeline panel. I noticed that when this panel first comes on screen, it is at the bottom of the primary Photoshop window, but I always move mine out of the way to my second screen. The space left when my windows are Fit on Screen, is the same height as this Timeline panel. So it's almost as if Photoshop still thinks the Timeline panel is there and won't fit the document on screen...it fits it to where it thinks the viewable screen real estate is. This is in CS6 for me, all updates applied.