This is normal. The Hired Guards are a special NPC setup just for Jammer's Gang to fight.
In order to change the NPCs that guard against your particular faction, you'll need to setup their Enemies. Part of it is explained in tutorial 4. The unwritten part of tutorial 4 you can set up, are "Enemy Only" faction control data objects.
To setup an Enemy Only faction control object, just create a new MiscItem, just like you did for your playable faction, but all you have to fill out is the FactionForm field, and the NPCArmy section, and then add that MiscItem to the formlist you created in tutorial 1 to register your data with Sim Settlements.
Then whenever your faction goes to make attacks, the defenders will be pulled from their enemy list.