Unfortunately this is a problem which occurs on many makes of Tablet PC (five out of five so far that I have tried anyway). The Acer is not one of the better ones. There are a number of postings on this forum about this problem. There is also a video from Michael Tsang of the Channel 9 team http://channel9.msdn.com/ShowPost.aspx?PostID=8549 (he wrote the drivers) which describes the problems with calibration.
The calibration is affected by the internals of the laptop and electrical components around the edge of the display. It is up to the manufacturer to shield these and to correct for any irregularities (see video above).
I have even found metallic objects brought close to the display and pen tip will distort the calibration (and even cause spurious right-clicks!). I discovered this when using a steel straight edge (backed with soft tape so as not to scratch the display) to draw straight lines over a grid to see how bad the distortion is. It is quite a revealing test but you need a steady hand and don't use a metal ruler!