Battlefield 3 - Roadkill in a Mobile AA?

So, I was playing a round of Conquest Large on the Russian team, on Caspian Border. I'd jumped into the Russians' mobile anti-aircraft vehicle and begun attacking a hostile attack helicopter. I ended up destroying the aircraft and killing its pilot (I don't think there was a gunner) with the vehicle's main guns, and the game gave me a 'Roadkill' bonus. (Note - This helicopter was a ways away, and there was no one around me.)

Does anyone have an explanation for this or has anyone else experienced this?