Archers I believe have to be able to put their cursor on centre mass as with all single target spells, so it needs to be on screen. Wizzies/tao's AOE however extend past the screen 1/2 spaces depending on the spell, so the wizzies aren't casting met/blizz on you, they are casting it next to you, 1 space offscreen, the spell itself is extending onto you.
Personally I don't think that extension is an issue, its a part of AOE mechanics, the problem is that you cant see the spell animations if the caster is off screen so have no idea your being hit by met/blizz, you just lose half your health to a blank screen. Im not certain when this changed either as I don't recall this animation issue was present originally.
