I think this is by design. When you move your mouse to the top/bottom corner, the charm bar is automatically positioned for fast access (on 4:3 screen)
Agreed. I heard about it in some M$ presentation (was it at Build?). When you come in from the top corner, the Charms will be aligned close to the top of the screen, and the other way round for the bottom corner. You only notice that on large screen resolutions.