What I have found is that Delta blocks (or not) seats based on fare class.
Flight 882 PDX - ATL with connection to DCA on M fare = seats 10B, 10C, 11B, 11D, 11E, 12B, 13B, 13C, 13D, 13E, 13F all blocked.
Same flight 882 with connection to BWI on B fare = all unoccupied C+ seats (except 10B & 10C always blocked until boarding time) available.
Same flight 882 with later connection to DCA on B fare = all unoccupied C+ seats (except 10B & 10C always blocked until boarding time) available.