"This is part of the design of the Dock: It's designed to not allow windows to behind the dock to eliminate the confusion of trying to access something in the window that has now fallen behind the dock.
However in order to not break resizing when the window bottom right is below the Dock's height position it will still allow you to resize the window and put it behind the Dock.
"Ahhh yes. The magical 'problems' of window resizing in OS X!
If the bottom of the application is 'below' or 'underneath' the Dock, then it will allow you to resize it even further underneath it. Otherwise, it will not let you resize it underneath the Dock.
I suppose they do this to limit confusion (should the click go to the Dock, or the application?), improve visibility, and to aid in Window management (so you can make all your windows the exact same height."
