I'm confused
 

I don't get it. I talk to pro snipers and they tell me to aim ahead but whenever I estimate I miss. But then when I match up the sniper crosshair with the body and shoot at that exact moment, I ALWAYS get a hit. Does aiming ahead really work?

You only aim ahead of them if they are running. This is called "leading".

Well that's what i do. Let's say they're just running in a straight line, I would study their movement then just follow em with the crosshair and when I think the time is right when the dot matches with the body I shoot and I ALWAYS get a hit.

Distance
 

What they mean also is the distance ahead of the enemy unit. Probably, in the games point of view, about a foot in front of them should be enough. Remember, this is in the games point of view, not your computer screens. You might be aiming(leading) too far ahead of the person?

just keep doing it how you are doing it as long you are ALWAYS geting a hit

What I do is aim to the head when they r like sitting ducks..they will always die with 1 bullet shot....now when they r running aim a little in front of them....becuase it takes a little time for the bullet to reach them...you physics?? lol...apply it..lol.... good luck