Years ago, NW
made the decision to "consolidate" their small DC9-40 fleet into the DC9-30 inventory and both types use the D9S code - thus, the DC9-40 can show up on any route shown as operated by a D9S. Its not even a supply/demand thing or a seat inventory thing, as far as I know, the handful of extra seats on the DC9-40s are simply not sold or accounted for in the NW
rez system.......the seats are used in the event of an overbooking or standbys, etc if a DC9-40 (instead of a DC9-30) happens to show up at the gate to operate a flight.
More typcially: DC9 = DC9-10/20; D9S = DC9-30/40; D95 = DC9-50