There's a lot of interesting answers and theories here.

The origination of the call can indeed vary on your bill. Your origination location is not based off of GPS, instead it's based off the individual cell tower you happen to be using at the time. Not a group of towers, or many towers, just the single individual tower you're using to place that call. Whatever county that cell tower happens to physically be located in determines your origination location.

The destination of the call is also determined in this fashion.