What units are you viewing the site in? If you take a look under the logo on any page there are four options: US, UK, L/100km, and km/L. The one you're currently using should be grey, the rest are green. Here's more info: How do I view Fuelly data in metric units?
What two fuel ups have you done? Also are you tracking by ODO or Trip, ODO omits the first fill up and starts tracking the mileage for the next tank. Make sure you do your math right also. Its miles/gallons.