1 We kind of have those ammo types. If you'll notice, the 50cals seem more likely to set something on fire than most other weapons. As I understand it (and I could be incorrect on this), the effects are averaged for each round, so instead of a mix of ammo types, we have 3 types. MG rounds, HE cannon rounds, and AP cannon rounds. There have been numerous discussions about this in the past, and requests to have historically accurate loadouts.
2 Lag. The game runs on every players computer, and you either see a slightly different situation than everyone else, or you see it at a slightly different time.
3 Lag plays a part here too, and also that we aren't real men flying real planes, but rather real men trying to control cartoon planes over the net, which means we don't have to worry about fatigue, tearing up an airframe, or dying.