It's possible they're displaying iPhone 5 mirroring (the iPhone 5 has a different ration)
Also, If the original content on the iPad is widescreen And your TV is widescreen this might be possible. (by playing with your TVs controls)
See discussion from earlier today.:
Hi Alix. Many thanks for you answer. As I suspected the performance of this product does not live upto the Apple advertising hype. It's a shame that I will have to adjust my TV every time I want to view my iPad and switch back to view Apple TV content. The advert showing a full screen is at http://www.apple.com/uk/airplay/.
I am very disappointed.
Actually Alison, every image in the ad you're referring to is the real deal. When you initially mirror your iPad to your HDTV, yes, you'll get the black letterboxed bars on the left and right while mirroring your home screen. This is the normal behavior because your iPad is not widescreen, and you would not want to s-t-r-e-t-c-h your image. But when you mirror media like video, photos and video games, AirPlay goes into a sort of "media mode" where it actually DOES fill up your entire screen. So widescreen movies actually WILL fill up the entirety of your HDTV screen. This is also true for games built for AirPlay functionality like the games shown in the ad (MetalStorm and Ducati Challenge).
Hope that helps clear up any confusion.