I got aether cloak tonight even though Gauthus told me I wouldn't be able to cast it... I can cast it, most of the time. I have casted 3 times successfully, and 3 times it has failed because I 'lack the skill.' I do lack the skill - my warding is 196, and my elemental is 255. My question is, why am I able to cast it sometimes?

There is a random component to spell casting success, and you rolled high a few times.

Mazrian

If you have a spare spell slot you can also pick up Warding Mastery. Around 180 in the given skill has seemed to work for me with that level of spell difficulty.

- Saragos