How busy a brigade is also depends on the demographic of the community.... Things such as average age of the community, income status things like that.
Our town is made up of more older people than younger ones and they tend to be more responsible etc etc..
I mean no offence to anyone living in the following areas but if you put the northern suburbs demographic of people in to say, Burnside then Burnsides call out rate would probably increase by maybe 50%.
I don't know if anyone could follow that but thats what i believe has a big effect on how busy some areas are to others.