No, I see what you mean. Most attack evocations do seem to "originate" with the caster - you see people throw fire or lightning or whatever all the time - but there are and will be instances of localised earthquakes, lightning from the sky, a wall of flame or powerful gusts of wind that don't start with the caster and end with the target. It's just down to how you visualise it.
To me, it seems like any evocation that creates something to attack with (fire, lightning, green bolt of entropy) always originates from the caster, while an evocation that taps something else (a gust of wind, lightning from the earth or a storm, gravity, seismic disturbance) can originate from elsewhere, and generally close by. That's how I remember it from the novels. Of course, I could be mistaken
At that point, targeting someone around the corner with a mirror and using an earth evocation to make the ground at their feet blast them with rock fragments is a perfectly valid method. Especially when you consider the importance of mirrors in many tales of the supernatural.
