How do people snipe so quickly on Halo Reach?

So I bought an Xbox and Halo Reach for the first time ever in my life a few months ago. I've been hooked on the game ever since and really fell in love with the sniper. I play Team Snipers a lot and got pretty good with the sniper. However, there is a threshold that I cannot pass from being an adequate sniper to a great one. Whenever I go against a great sniper, even though I have the first sight of him, as soon as he sees me he usually gets an immediate headshot; sometimes while even jumping. Now, I am ok if he gets lucky once or twice, but a headshot almost everytime not only to me but my teammates as well does not make sense. I strafe and move around pretty well, but the good ones always are able to headshot me first. For example, an opponent is running toward one direction not looking at me, and I get a body shot, he immdiately turns toward me before I can even get a second shot and gets a headshot. Like I said, if it was once or twice I'd be ok, but many many times? How is that possible? I feel like there is a threshold that I cannot pass. I have learned sweep sniping, leading my shots, not staying still in one position, strafing my shots, and aiming for the head. I am already good with the sniper if I camp in an advantageous position, but what I really want to learn (what separates the noobs from the pros) is how to shoot well quickly. I use a green thumb controller figuration and 4 sensitivity. I have tried different sensitivities and this one feels the most comfortable for me.