It happened on mine too, so I don’t think it’s the web host, etc. As best I can tell, it’s been designed for a different video aspect ratio. The demo video is maybe 2.35:1, not 16:9. Most HD video is 16:9.
I have the same problem and while I have poked around have yet to figure it out.
Please update us on what function controls the video aspect ratio on the slider. Thanks!