My observation is that if you get a different animation based on what end of the range your damage roll ends up in. If I get a bad role and get the low end of the range, I get animation #1; a good roll and high damage gets me another animation. I haven't kept track of how many different animations I've seen for a spell. Maybe one of the moderators can chime in. :)
I've seen up to three different animations for the same spell, and the poster above is correct, it depends on how much damage you do. Some only have a high and low animation, others have a low, mid, and high damage animation.
For example, the Snow Serpent spell has three animations. The low damage one shows the serpent spit a chunk of ice at the target. The mid damage animation has him twirl around the enemy creating an icy vortex around it . Finally, the high damage animation has him recoil and then strike the enemy with his vicious bite.
The Cyclops also hase three, though I dont recall which is for what damage range. I know one has him splitting the ground underneath the target, another has him throw his hammer at the target, and the third has him actually jump towards the target and strike it with his hammer.
You have to just look closely cause sometimes the difference in the animations are subtle, but they are there to make them less repetitive. I would have liked to see some different animations for the different element types of the same spell. Like the Snow Serpent (fire element) should spit out fire instead of a chunk of magma? I guess. That would change things up even more, making battles less repetitive.