I just assumed that the trigger to call for the armor reinforcements was delayed until either we cleared the town or when we started to move toward the Primary Objective. We had a Waldo who was hiding in one of the cabins (which you can see in the pic above) that we didn't find until the last minute that triggered the Objective Complete status and that's when we started to move in toward the Primary Objective. The armor appeared just as we started to move, so that's why I assumed that the armor reinforcements were set to trigger when we either cleared the town or moved in to a certain distance around the Primary Objective.
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"You know what the chain of command is? It's the chain I go get and beat you with 'til ya understand who's in ruttin' command here."
- Jayne Cobb, "Firefly"