Sadly that'll change after you drive for a while. It calculates the range based on the most recent data it has about your consumption AFAIK. So if you were getting really high mpg figures just before you filled up then I suppose it's think you were going to carry on like that.
I always enjoy the game where the DIS says you've got 40 miles to empty, and you drive like a saint for your journey to work and arrive there with 55 miles left.
I know what you mean, 1000 miles would be epic. I'd only have to fill up every 6-8 weeks!
You've done pretty good with 687 though.