It's just lag. It happens all the time to everyone. Sometimes if the lag is really bad almost anything can happen. So it's not that halo is broken, it's just that some people have crappy connections. Hope that helped.
Reasons: 1. Lag 2. Standoff = big map = more lag then say guardian. 3. Here's a biggy. Sometimes when people are right up against a wall, rock, etc. They can just see over. These places are usually at different elevations to you. They can shoot you from these places, however, you can put 4 rounds of sniper into their head and not make a scratch, even though there is zero lag and blood lunging out of their brains. I found a lot of these when playing split-screen/LAN (therefore no lag). I expereienced it happening sometimes at the rocks on standoff (the pairs on the ends of the rows infornt of bases, opposite ends to rockets.) were people crouch behind the smaller rock and survive what would be regarded as overkill. This has also happened to me on highground laser tower. I can't confirm this scenery clipping theory is true, however, it seems to happen.