How is it possible EA ?

I believe mortars are classified as damage dealt by a vehicle; and mortars are famously useless in this game, dealing damage but rarely kills. Maybe that's it?

... at least I know that I have accidentally completed some vehicle challenges while using a mortar. So, at least at some time, the mortar was classified as a vehicle.

I've heard, but haven't seen confirmation, that this is due to a glitch that counts damage dealt by exploding vehicles as damage for the player.

For example, you shoot a helicopter down, you get damage for the helicopter, and then on top of that, you get damage for everything the helicopter crashes into (buildings, players, other vehicles, etc) and destroys.

Now, once again, I haven't heard this confirmed, but this would make sense if the player spent time shooting down aircraft that kept crashing into buildings and demolishing them. Could also be completely made up (not by me, I have heard others talk about this). Who knows?