I hate to break it to you both, but Halo isn't even a remotely realistic shooter in terms of mechanics, and GGO clearly is trying to be, laser guns and swords aside. Halo is also one of the "slowest" shooters when compared to most PC shooters. That makes it much easier to aim and shoot a sniper rifle from the air. Not to mention the game has auto-aim because it is on consoles.

You'd never be able to do that in Unreal Tournament without an aimbot. Hell, you can barely do it in Fallout (can't use VATS in midair).