Why are the maximum X and Y touch coordinates on the Surface Pro is different from native display resolution?

Question

I have noticed that the Surface Pro and I believe the Sony Vaio Duo 11 are reporting maximum touch coordinates of 1366x768, which is surprising to me since their native display resolution is 1920x1080.

Does anyone know of a way to find out at runtime what the maximum touch coordinates are? I'm running a DirectX app underneath the XAML, so I have to scale the touch coordinates into my own world coordinates and I cannot do this without knowing what the scale factor is.

Here is the code that I'm running that looks at the touch coordinates:

// From DirectXPage.xaml
<Grid PointerPressed="OnPointerPressed"></Grid>

// From DirectXPage.xaml.cpp
void DirectXPage::OnPointerPressed(Platform::Object^ sender, Windows::UI::Xaml::Input::PointerRoutedEventArgs^ args)
{
   auto pointerPoint = args->GetCurrentPoint(nullptr);
   // the x value ranges between 0 and 1366
   auto x = pointerPoint->Position.X;
   // the y value ranges between 0 and 768
   auto y = pointerPoint->Position.Y;
}

Also, here is a sample project setup that can demonstrate this issue if run on a Surface Pro:

http://andrewgarrison.com/files/TouchTester.zip