Can someone explain LOS to me?

[SlouchyBard]

LOS stands for "Line of sight" so essentially if you can't physical see it from your own characters eye's, you aren't gonna hit your target. Its like throwing a ball back and forth, you can't just throw the ball through a wall.

But to answer your question, I am guessing it could be "lag" or possible "slide casting" where the actual cast goes off which is before the animation is done. So it could be possible that the game registered their cast going off before the game registered you behind the wall.