Wow Ped...that answers the same riddle for me!!!!!! THANK YOU!
I've been trying to figure out why I can (tank) shell a plane sometimes but most of the time it seems to explode harmlessly (like it hits the plane but does nothing). I figured it was the location on the plane that I needed to hit. Like hitting the wings would take it down but the engine wouldn't. But, I think you are right in that the plane shot down the shell! So simple and I never thought of it!
So, now I need to keep firing when approaching the tank as opposed to waiting until it's directly in my sights in case it fires. Also explains why I've never seen a "mis-hit" on the side of a plane or the back of a plane. I feel so stupid.