Yeah, it has everything to do with locking the cursor to the window. The cool thing about a tablet is how "this point on the tablet is always this point on the screen" which makes it really easy/natural to snap the cursor around. (well, at least for those of us who grew up holding pens and pencils in school...) However, it also means the tablet keeps telling the game "cursor is over here" making the game think "oh, the person must have moved the cursor all the way from the locked position to there. Now I'll re-center it" after which the tablet again says "the cursor is over here" and the cycle repeats, making the game register HUGE cursor movement when you're actually holding the stylus in one place.
You can tell your tablet to use relative tracking for the stylus, (or use the mouse, which generally tracks that way,) but then you lose the thing that makes it so great in the first place. So with most games you usually just end up putting away the tablet.