Originally Posted by
SPARTAN VI
I found the answer to my question.
You can't modify individual cities' garrisons. There are multiple entries for major cities because they need to be able to specify what garrisons those buildings provide in the event another culture owns that building too (e.g. Rome conquers Carthago, but doesn't convert the building; it will likely get a few plebs and rorarii instead of a full-fledged garrison). There are also additional entries for every major city due to multiple unit groups assigned to such a city (e.g. a ranged unit group, melee unit group, and mob unit group). Furthermore, some buildings will have duplicated entries for unit groups simply to double/triple the amount of units it provides as a garrisons, which is particularly common with upgraded buildings (e.g. a basic melee unit group will have 2-3 entries to double/triple the amount of units it contributes to the garrison).