When you fill up, the range will be calculated based on the average mpg for the last few miles, and will assume that you will continue driving like that. Trundle along a dual carriageway at 40mph for 20 miles, then fill up and could could well see a potential range of 600+ miles. Drive it like you stole it, then fill up and the range could be as low as 100 miles, but drive economically after than and the range to empty figure will increase.
I think that the feature is pretty pointless until you have below 1/4 of a tank of fuel and are miles from the next fuel station.