This is a difficult but good exercise. After spending hours trying to perfect my spin curve, now I'm looking at the example gif and it looks to me like the ball is spinning way too fast for the movement. It looks like it's spinning out, and is maybe 1.5-2x faster than it should be. I can't not see it now. Arrggh! Is what I'm seeing true, or am I seeing things? :)
This exercise will probably drive you crazy because your eye will switch back and forth between it being "perfect" when you play it once and then "totally wrong" when you play it again.
This is just your brain learning to see and telling you what it thinks you want to see. Make sure you rest your eyes and come back to it as your brain will start to strengthen the nueral pathways with myolin. You need repetition and rest for this to happen.
Don't worry - it will get easier to see, you are at the most difficult point at the moment.
As for the gif, I can tell you that the 'animation' is spot on, however, the frame rate of the gif is not consistent with the frame rate it was animated at. (I was not the one who created that, but I think it's just a screen captured gif of the animation)
It's good that you can see something weird about it - it means your brain is learning to see :)