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While i cant confirm whether this is the case, i believe they have the ability to choose which talkgroups are carried on both zones and which do not. Its possible they have the north zone setup to only carry talkgroups designated as north ones (or close to the north zone) and ones considered countywide talkgroups. You would hear Zone 6/11, north band, etc, as well as zone 9 (due to proximity) but not zone 1, 8, etc.
One step from "Banned For The Greater Good" lol