It was discussed a little more in depth a few years ago, but IIRC, every 50 cal round is a combination of AP, I, and HE, so that some rounds upon striking would just make holes, some would cause more damage, and some would touch off a fire, with the goal of simulating the appropriate mix of ammo types without modelling 5 different types of ammo for a single loadout.
Not 100% certain, but I think that was the basic premise/theory. I don't know if it's randomized like oil or fuel hits, or if each round has a fixed chance to yield whatever result, or how exactly it works.