This is probably caused by the shape of the tank more than anything else. When you fill the tank, it's not just full, but the filler neck will also be full - it takes 20 miles or so to use this extra full, so the tank stays 'full' for that period.
The range itself is calculated from your recent average, not the overall average mpg. If you drive really carefully for the five miles before filling up, your range after filling will be artificially high
Hope that helps, Jon