Question about ranger AoE

[Chinetter]

My 22 ranger now and then has occasion to use his little lightning-based AoE, which does 25 dmg per mob and is only rarely resisted by any mob.

I have noticed though that the spell also targets the guy who cast it. Sometimes I get a successful resist but sometimes not. Also I notice that unlike the mobs, on whom the damage when it lands is always the full amount, on the caster the damage taken ranges from 5 to 25.

Is that how AoEs are supposed to work, the character casting the spell also gets hit with it? Or does the above mean I am doing something wrong in how I deploy that spell?