If the docked control bar is configured to be always visible (controlBarAutoHide=true), the available area to display the movie is reduced. You could also see sidebars in background color, or a change in aspect ratio.
If the control bar is set to be always visible, the playback area is reduced by 35 pixels (the height of the control bar). The movie is then scaled to fit into this smaller area. To compensate, increase the height parameter by 35 pixels, to increase the size of the playback area.