As Zup said, its all about physics, and the bullets ARE objects that, like any other, interacts. I noticed that, if you aim right at the middle of the plane, chances are that the enemy's bullets will blow the tank shell in the air before it hits the plane.
By the way, I remember playing against Cptn Noushbag, tank versus tank, we were just few metters from each other, and we managed to fire at the same time, and our shells aparentely hit each other in middleway. It happened with me another time, I believe it was against Parrot Head, if not wrong. Yeah, twice... I used to play a lot in the early years... lol
And in other game, I was defending my carrier against a swarm of reds, and I stongly believe I managed to shoot and hit a bomb dropped from a red plane in middle air, right before it land on my ship... A great save. Of course, at the pace of the action, I can not be sure if it really happens (cuz it could be, for example, a lag explosion against a misplaced laggy plane), but even the red attacker said something like "wow, you blew my bomb in the air".
Tip: when trying to hit planes from tank, if it comes at your direction, try aiming a little aside, for the wings; and if the plane is not flying towards your tank, try aimning at its "future point", a little ahead of what normally should be, to compensate some lag. Ir works for me, and even these days I keep killing many planes from tank almost every game.