Overdrive Options

So, let me get this straight, the a500/a518/42rh/46rh are driving the OD all the time even though not in OD therefore pulling HP, yes? I just don't see it. Why would Mopar build a trans meant to help improve mpg yet rob as much as 100hp? That would mean it wouldn't be good for mileage over a normal 904/727 even with the OD! I would really like to hear from a Mopar trans tech on how the OD trans works to see if this is actually what is happening.