Jamieson, Fausset and Brown on Jeremiah 39:9

9. remnant--excepting the poorest (Jr 39:10), who caused Nebuchadnezzar no apprehensions.

those ... that fell to him--the deserters were distrusted; or they may have been removed at their own request, lest the people should vent their rage on them as traitors, after the departure of the Chaldeans.

rest ... that remained--distinct from the previous "remnant"; there he means the remnant of those besieged in the city, whom Nebuchadnezzar spared; here, those scattered through various districts of the country which had not been besieged [Calvin].