I think what it comes down to is a NA engine makes its torque peek high in the RPM range you need higher numerically gears to keep it in this range. With a turbo you make a broad flat torque curve when under boost so you load the engine with numerically lower gears to force the turbo into boost. In other words you are cruising along with no load and no boost, but you put a trailer or climb a hill you start making boost and thus more torque.