The following factors can have orders-of-magnitude effects on the costs of the last mile.
* Cycle Times (the time from route start to route finish). If you offer short cycle times, you lower your delivery density (distance between deliveries).
* Market Share. The more customers you have, the higher your delivery density.
* Range. The bigger the delivery area, the further you have to travel at the beginning and ends of delivery cycles. Short cycles with large ranges can add up to very large dead weight losses.
* Street patterns. Street patterns can ease or severely inhibit efficient routing. In a town full of cul-de-sacs, you have almost no flexibility to optimize your routing. In a pure grid, you can optimize extremely well.
* Vehicle requirements. Smaller vehicles are extremely nimble, aren't subject to the same traffic considerations, and don't have problems with parking, ingress, or egress. Larger vehicles can be much more difficult to move around. This is actually a huge benefit to companies like Taobao, and pizza delivery, because they can use motorcycles effectively. Refrigerator Delivery, not so much.
And to compound that, labor and fuel costs are a multiplier for the factors mentioned.