I don't think it has anything to do with your processor speed, or the speed of your internet connection. It has to do with when the server updates your client with his position.
On your machine, you're sitting behind the spawn hangar, with no enemy in sight.
On his machine, he spawns in. This required interaction between his machine and the server. But you don't automatically know that he's spawned in - the server has to tell your game client that this guy has spawned in, what vehicle it is, his current location, speed, heading, etc. There is some time between when the other guy actually appears in game, on his computer, and when the server ends up telling your game client that he's there, and in the time in between that, he's in a T34 (always use a T34 for this) rotating his turret like mad. If the delay is long enough, he's already got his turret on you before you even see he's there.
I've been killed by guys in a spawn hangar before when they still weren't visible to me. A shot just appeared out of thin air and killed me. In fact, I've had a guy fire on me out of thin air, before my client knew he was there, and in a knee-jerk reaction I fired back, and since my game still didn't think he was there, my shot just passed through his location and did no damage. He appeared an instant later and fired again, killing me. Yeah, it's frustrating, but that's the way it goes.
One thing that helps explain a few of these wierd little things is that this game operates under the principle that what you see in the game on your computer will affect you, and what he sees on his end of the game will affect him. If your game doesn't see his tank in game, your shot won't hit him, even if on his computer, he's really there. I have heard that they programmed in a little random delay in updating others of a new spawn-in, in order to make it easier to break a camp. I don't know if this is really true or not, but if it is, just add that little bit of delay to the delay due to lagtime between his actions and when the server notifies you of his actions, and the principle remains as I explained it.
This works in the air too. Your position, as you see yourself at any given time, isn't actually the same as what the other guy sees on his machine. There's always a little lag time between when you make a sudden control input, for example ( that reacts instantly on your machine), and when the other guy sees you do that, so it can be rendered on his screen. Doesn't matter. If he shoots and hits where his computer thinks your plane is at that instant, you'll get hit, even if on your computer you'd just juked a little and he would have missed you. When you shoot, your computer will hit his plane based on where your computer thinks he is, not where his computer thinks he is.
This also explains why sometimes you'll get a collision message and the other guy doesn't, or vice versa, when it was a head to head thing that should have collided both. What's happening is that on his machine, some last-second maneuver caused him to barely miss you. So since on his machine he missed, he doesn't get a collision. But since on your machine, you didn't see his last-second maneuver, and you didn't perform one of your own, so your computer saw you hitting him, and you get a collision. This doesn't really sound "fair", but overall, the scheme works insanely well, and makes it so that even with the lag between your actions and when everyone else finds out about them, and vice versa, everyone sees things as happening smoothly and responsively on their own machines.
There's one exception I can think of to this "what you see is what you get, and what he sees is what he gets" rule. And that is landing. If you shoot a guy, and on your screen you clearly see the hit, but at that exact same time on his computer he's just towered out and is safe, then he'll remain safe, and the server will reject your hit. I've seen this a few times, most notably with a set of B-17s that landed on a field I was camping in a T34. I fired an HE round that I clearly saw impact the cockpit area of a B-17, which should have given me an instant kill, but at that same instant, the B-17 pilot had just ended flight, so his planes disappeared and I got no kill. This same thing happen with an M4 whose turret I had knocked out at a vbase I was attacking a couple days ago. I fired a second round and hit the guy right in the side from 300 yards out, which should have killed him, except on his screen he'd just ended flight, and so his M4 disappeared unkilled.
I hope this helps. Most of the time this mechanism is invisible to us players, and the illusion of everything being updated instantly is maintained, as if there were no such thing as packet latency and server processing time and whatnot. Every once in a while something wierd happens, like tank shells coming at you from thin air, and it helps put these in perspective, if you understand the mechanisms and why these things happen.
