Why does the Wii U display websites weirdly?

So I was messing with my Wii U's web browser and found their cool little developer tools feature. I enabled it and loaded up my website.

While inspecting it, I found something weird - the website's body displays as 980 pixels wide, even though the gamepad is only 854 pixels wide according to Wikipedia.

If it matters, here's a link to my site, but I'm pretty sure it's not my code that's doing this. Any ideas?