When it comes to astral projection, the why's and how's are purely theory. I don't feel there is a point in arguing speculation.
Psychologists and science will state that it is indeed your subconscious that creates the scenes and you are experiencing nothing more than lucid dreaming. Mind you, they have no "proof" of this explaination. However, in a spiritual perspective, it is believed to be your spirit/energy that wanders, driven by your conscious mind. I think science likes to try and have an "rational" answer for everything to a point of grasping at straws in the case of many spiritual things, including AP. Try getting a scientist to say "I don't know". It doesn't happen! =)
However, I do disagree that the soul goes anywhere. If the soul leaves the body, the body dies. The soul is our life source and is the reason why the "rope" technique works. Your soul is your anchor. Ropes aren't necessary, only a way to make those new to the experience feel more comfortable.