This is due to the order in which all players move. First the AI, then the human player. When you cast the spell, the AI has already made it's move for this turn. This means, that the spell has no effect in this turn, but it still counts against the duration of the spell. The next turn works as expected and the AI is free to go at the beginning of the third turn. In essence, you prevent the AI from moving for only one turn.

There are two ways of fixing this. Rearranging the order in which everybody moves (highly unlikely at this stage of the beta) or increasing the duration of the spell by another turn while keeping the description as is.

Updated post to reflect the fact that this also applies to freeze, and persists in the latest version.
With a five-turn cooldown on freeze, it is especially useless at buying time. 

This really needs to get fixed. It was one of the first bugs I reported back in May and it is still in. Just increasing the internal value for the spell duration by 1 would solve it.

They ought to at least correct the description if nothing else. If you want to go a step further, how about reducing the cool down on freeze? Seeing that it only immobilizes for 1 season, 5 season cd is pretty bad.

This is only a problem IF the AI uses it against you... but they don't. If they did, you could make an argument that the AI gets benefit for 2 turns, but you only get it for one. But seeing as how they don't, the only question is whether Tremor of Freeze is balanced, or underpowered and needs that extra turn of effect. The answer, IMO, is that they are fine as they are (and I use Tremor a lot), some might even consider them OP. Yes, you do have to cast it every turn to keep an army stuck. But for the cost of 20 mana per turn, you basically have the ability to COMPLETELY disable their strongest army. I can't see any reason why you would argue that it's too weak and needs help.

Freeze has a long cooldown, but it isn't meant to be used like Tremor to stop an army, its use is more like Wither, where you use it to weaken the army and then attack it with an advantage.

I suppose you could just alter the description to say 1 turn, but it really is 2 turn of no movement (it just so happens that the AI would have already used one of those movement). So maybe something like "Stops the army from moving next turn" would be more accurate. Still, it's a pretty small issue.

Hmm, Im unable to repro this in the manner it is stated.  For me when I cast tremor on an AI unit it says that they are immobilized for two turns.  When I click end turn it shows that they are immobilized for 1 turn.  Then when I click end turn they aren't immobilized they can move.

So the effect is that they only lost one turn of movement.

I could increase the duration to 3 to make the spell more powerful.  I could increase the game effect to 3 but keep the desc saying 2 so that it is less confusing to players that are expecting the unit to lose two turns of movement (though that will mean they cast it and see "immobilized for 3 turns" after casting a spell that was supposed to immobilize for 2.  Changing the way the turn order happens is a bad idea at this point.

Have to think about this a bit.

Ach! Unclear turn ordering strikes again! I vote to make each enemy have their own turn, in a clear order! [/broken record]

I have to agree with that, though.

Still, when it says "Immobilizes for 2 turns" and the enemy only loses *one* turn of movement, that's confusing. People are used to playing board games, where if it says "Lose two turns", the player loses their next two turns. It's really quite counterintuitive that "Immobilized for two turns" means "Immobilized for two player turns, which means only the one AI turn in between."

This is what I suggested during Beta.  You must have been sick that day and didn't read my post. 

Or perhaps tweak the code behind the spell, so that it checks if you've already used all your movement. Such that, if you already moved (cast on an AI, move == 0), the effect is applied starting on the next turn (or just add one to the effect), and if you have not (still have movement left), then the effect starts on this turn (like it does now).

That way, if the AI ever learns how to use it, it'll still be fair.

Thanks!