I remember the old Nintendo DS touch screen, which was resistive, required user calibration where you would press on the corners of the screen. If you missed the corners (which, with the relatively thick stylus and the relatively thick bezel on the original DS, wasn't uncommon), the accuracy of your touch positions would be very notably off, to the point that using touch-screen keyboards in some games would be difficult.
Obviously I don't know for sure if this was the exact case with this voting machine, but the point being, some touch screens do require calibration and there is a chance for error. Or, of course, it could simply have been a factory mistake.
Either way, I doubt there's any malice behind this error.
Multi-touch screens as used in smart phones use capacitive touch sensing technology, they do not require calibration.
Cheaper, older touch screen systems (such as those used with ATMs or voting machines) use resistive touch technology, they absolutely require calibration.
2. Because these machines are open to the general public who are hitting the screens with their meaty fingers all day.
I have used a ton of touch screen devices in public. I've encountered quite a few that were miscalibrated.
Public devices put up with abuse that your private devices do not.
My kids abuse my iPad pretty heavily but generally smacking on the screen with their hands and fingers a lot but I still have never seen it mis-calculate a touch.
I don't know much about electronic voting machines but I wouldn't be surprised if they mostly used resistive touchscreens (which are historically much cheaper to produce).
I recently had to help my mother recalibrate her sewing machine's touch screen.
If you've ever interacted with a touchscreen at an ATM, mall kiosk, or airport kiosk, it's likely that they were resistive or infrared based, and more susceptible to calibration errors.