It draws a B-line from the start point to the end point of the tween.
There are some ways you can fix this. You can add a mid-point to the tween, to make sure the hand travels more in a curve shaped pattern, but it still wouldn't be exact.
Most of the good flash animators do frame-by-frame animation, with tweening & the other effects used every once in a while. Go to flashkit.com for tutorials/resources while you're at it.