I suppose at some level they have to calculate the response time and available units. Units tied up applying vasoline to handcuffs, are available to get cats out of trees. The operating cost per mile of fire equipment has to be astronomical, probably on the order of $10/mile, or more.
They could be over-responding to make themselves look busy. In one town where I lived, even the simplest 911 call got you police, a fire truck, and an ambulance.