this should be the "optimal" orientation based on their mathematical model, it'd make sense because it accounts 1)the fact that vastus lateralis is more activated compared to the default, "max inertia" position + maximal hamstring activation, which is the higher end torque phase and 2) it's the phase of maximal rate/increase of torque and not just the resulting maximal final (with a cranck at ~108°-110° or ~18-20° under horizontal crank compared to ground).