With my experience on DCS, when passengers loads are light, it is better for weight and balance purposes to let the system allocate the seats, and not to manually assign them.
the DCS is programmed with all the relevant data for that particular aircraft, and therefore will plan its most economical trim, based on all things planned/booked for the flight- passenegers, baggage, cargo, mail, etc.
can't explain why pax don't take other seats after airborne though - stupidity?