The "gap" is a "feature" of the cheap Wacom digitizers - i.e. the ones that get built into Tablet PC's (and iLiad). It isn't caused by software, it isn't caused by (mis)calibration, it isn't caused by the angle of the pen or any such thing. It's caused by intereference / layout errors in the digitizer grid itself.
It's present in some cheap Wacom tablets too (Graphire/Volito/Bamboo), although it's usually less noticable. It's always near the edge of the screen/tablet, which makes it quite a nuisance when you try dragging a scroll bar (and end up resizing the window
).