This discrepancy kind of happens in almost every LOB. Buying more usually lowers unit costs. I could even argue that those that trade in futures (grain, fuel, etc.), are trying to adjust the "time dimension" of their use to lower their cost which isn't that far removed from time-sensitive UBB.
Now before I get slammed, I don't agree that the economics of an ISP are in any remote way similar to the economics of other lines of business that lower costs through volume or futures purchasing. I'm just recognizing existing examples of lower costs for more vs. higher costs for less.