I am sometimes able to shoot over/under shields, too. I dont think that is lag. I think the engine compares the shieldskill with the archer-skill and so the arrow hits his target. I see it as some "Chance for luckshot", that is coming with higher level/skill. it happens maybe 1 of 100 hits.
On the other side some of my arrows are hitting the shield, while i am shooting in the back of the shielder. Like a 360 degree-energy-shield-bubble. That is, in my eyes, the special-skill of the shieldskill.
if you understand what I wanna say. 
It's just that the magic magnet of the shields is not as big as it used to be and hasn't been for a while to my knowledge, meaning the lower the shield skill and the size/shape of the shield your target has, the more chance you have of shooting over, under or around the shield.
With 5 shield skill and the Elite Cavalry shield I can be shoot in the foot, but it's very difficult for someone to pull it off, especially at long range because I'm mostly always moving. There isn't some kind of engine based skill comparison, it's all based on your own personal skills. Unless of course your crosshair is the size of your opponent, then it's just luck whether you hit his foot, lol.
