Fire Department units are dispatched according to information received by the 911 operator. We base that response on the worst case scenario based on the information in hand. The fire department's philosophy is to get our firefighters there as soon as possible.
There may be three or more fire department vehicles on the scene for what appears to be a "simple" incident. However, in emergency services we have learned that if we assume something is "simple," we can be horribly mistaken. The winner in these situations will always be the citizen who needs help.