Most of the larger munis have lower numbers like you are observing. Some assign them as they are hired so low numbers like a 2 digit is someone who has been there awhile. O'Fallon use to call their officers "ocean units", East St. Louis is/was "Edward units", Alorton was "Adam" you get the picture. Cahokia use to be called "Charles units". One Cahokia officer that came over when they merged with Centreville is "Charles 5" because his Centreville PD number was "5" he still calls over the radio like this.
Occasionally if they go to another department, they keep part of their number. For example if an officer was at Cahokia and was 117, and they go to say Fairview, they might keep that DSN if it's available or he/she might become 217 or 317. If they go to a smaller muni they might get the agencies 2 number identifier plus 17 like 6017 (Millstadt). I've been listening to this county for a long time, I know when officer's move by their voice and they're somewhat similar DSN/ID.
Fun fact, most, but not all St. Clair county PD units first 2 digits is the last 2 digits of that cities zip code. 39xx: Dupo (62239), 40xx: East Carondelet (62240), 60xx: Millstadt (62260) etc
Check the wiki and maybe help update it as you hear units.
