My assumption has been this: I have been surprised by the fact that there seems to be very little very little attention paid to keeping the additional payloads in the Center of Gravity. The Phantom design takes liberties with COG that fixed wing birds wouldn't think of. I am primarily talking gimbals and cameras here. They are hung under the front of the bird....to eliminate landing gear in the FOV of the camera.....throwing off the COG. The fact that it works at all, I assume, is that the various balancing mechanisms, reading the levelers, compasses, accelerometers, etc. correct for the inevitable tilt and adjust rotor power to achieve level.
BUT, preparing to takeoff from a level base...the bird has NO WAY of knowing what the weight distribution is, and how to adjust various motor powers to balance UNTIL it actually lifts off. If the COG is to the front, it initially powers all motors equally...the bird starts to lift....then lists to the heavy side...inevitable starts to side that direction...then the balancing mechanisms take over and it flattens. Actually, the exact same argument can be made about propeller balance, although prop weight balance does not translate as directly into reduced or increased lift, but LIFT balance between props is another unknown UNTIL the bird actually loses contact with the ground and begins to tilt...and inevitably slide...then corrects.
My bird slides forward on liftoff most/all of the time and m assumption has been because of the Off-center COG. I could be completely wrong in my analysis but this I what makes sense to me.