I have only been playing COD for about 4 months but I have noticed you never see two people both die when shots were obviously at the same time. Only one dies. Its as if one bullet becomes dead and the other does not. They need to fix that.
I believe thats called "Lag Compensation".
A lot of times its that one person was just more accurate than the other. They get they kill shot while the other just misses or wounds.
