I just got what's wrong here.
The animation as defined on the sample happens perfectly, *IF* the cursor is stopped and doesn't move after it first enter the element that triggers the tooltip.
Since there is a huge delay between entering the element and displaying the tooltip, the cursor can move to another random position on the element. If you position the cursor in the middle of the element before the tooltip displays, it will be animated almost always on the correct place. But the random reposition that happens after the animation is not so random at all; the tooltip gets a move back to where it should be if it were shown without delay, the position where the first mouseover event happened.
The solution to implementing this animation correctly (as it seems to me) is to track the changes to the cursor position and update the animation position relative to the last cursor position captured while ignoring the position of the event that actually triggered the tooltip.
Of course, this tracking should be done only when options.track == true. I've been trying to write my own (...*cough* crappy! *cough*...) fix for this, but it seems it will require changes on both jQuery and jQueryUI.