Just guessing based on how the technology works, but I would presume that when you initiate an emergency call via a cell phone call, the best location indicator that emergency services receive is "somewhere near tower X", since you are just making a voice call.
Apps running on the phone have access to your phone's GPS and can get more precise location data, if you have allowed it. Using an emergency app (Active911, Smart911, etc) will provide more specific location data to emergency services, but these are applications that rely on data and GPS to be working and may not work in as many situations as a simple 911 call.
As far as who receives the call, I imagine it is just based on who the tower is preconfigured to forward the call to. If the tower you are connected to is close to a highway, you'll probably get highway patrol. If you're in town, you'll probably get local police.