Chris is exactly correct on this. This is what I was trying to explain because Verizon is pretty much known for using 850 MHZ everywhere which won't hardly penetrate a house let alone a metal case. Then you have at&t that uses 1900 and will penetrate cement and other very hard and thick objects. Verizon has gone with the first method because the signal will travel further giving them better coverage in most places, At&t is just the opposite. So neither is bad, just bad for case manufacturers for the Verizon phone. That is why I was so impressed by what Element was able to do with their case!
You're right on your concept, you just have the frequency properties reversed. 850 is the "more penetrating, longer reach" frequency. When using this frequency, the towers can be spaced out a lot more, because of it's greater coverage strength. 1900's less penetrating, lesser reach properties means the towers have to be spaced closer to cover the same way. It's why 850 is typically used in more rural areas because it's reach is further, as long as the carrier has a license to use it in that area.