Yeah I studied physics too and took statics and had the same initial reaction as you. What you're missing is though the cable is directed to the middle of the board, the cable starts at the top of the highback. Would it be more efficient if the cable ran down towards the toes? Yeah. But being that you have to drive the force through the whole length of the highback on a 2 strap you still have better energy transfer with Flow.
Also my experience backs it up. Flows are more powerful transitioning edges. It's noticeable every time I ride both in the same day.