Think of it like this: If the front is heavy it is basically pulling the rear along with it, especially through flesh, etc. There is no "battle" to lead, so to speak. If the rear is just as heavy, or heavier to use an extreme example, when the front encounters resistance, flesh, then the rear will try to "swing out" and proceed on its own. Then you do not have the maximum energy directed at the tip/front, it is now being spread out across the shaft, thus decreasing penetration potential.

That is my hacked up description in trying to regurgitate what I have read. My high FOC arrows punch through my targets all of the time and my lower FOC arrows are less likely to. So, even my experience has shown me that high FOC out penetrates low FOC.