Can someone explain TTK to me?

I've experienced the same exact thing. I have games like the other day where I have Purpler armor and the other guy has blue. I had the level 1 extended light mag. R301 full clip in the Gibraltars back. Didn't miss a shot. He turns around and kills me before I finish my reload. Crap like that happens every now and then. It is frustrating. I've had it happen the other way around 2 or 3 times. Felt like I was cheating. I was melting people with the Havoc half way across the map. I was like how did I just kill that guy.   Have games where the hit reg and ttk seems perfect. I actually can react and have counter play before they kill me. Then other times there is 0 I could do in that situation to stay alive. I've had people kill me on wraith that I hit her ability right as they first shot me and I'm dead before I phase.