The fire departments in most towns in America provide emergency medical services. 911 calls about health related things often result in a fire truck arriving, rather than an ambulance.
You've got hospital ambulances, private ambulances, city fire departments, county fire departments, volunteer fire departments and federal fire fighting organizations all potentially in the same place, all summonable by a 911 dispatcher. They all have EMT training and gear. Who shows up and how is it decided? Beats me.
Sometimes human organisation is baffling if you are seeing it from a step further out but makes complete sense when viewed from the inside.