Never worked with tablets (Edit: sounds like some virtual desktop or something). For anyone who has you still need to give more clues. How do you determine the windows/desktop/backbuffer size, i.e. what function call do you use ? Also how do you grab the pointer coordinates ?
And another edit: Do you setup the device in fullscreen mode ? I also wonder if the pointer coordinates do have the higher resolution - hardware-wise.
It is a windows store app (metro) so windows is supposed to handle all that. Windows decides that this device is this resolution, this other device is another resolution etc.